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Columbia  (Bmtotttfftp 

in  tlje  €f  tp  of  I^rtti  gcrk 

College  of  ^fjpsictanfi  anb  burgeons; 
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GIFT  OF 

Frederick  S.  Lee 


Digitized  by  the  Internet  Archive 

in  2010  with  funding  from 
Columbia  University  Libraries 


http://www.archive.org/details/anatomyofnervouOOrans 


THE   ANATOMY 

of  the 

NERVOUS    SYSTEM 

FROM  THE   STANDPOINT  OF   DEVELOPMENT  AND  FUNCTION 


By 

STEPHEN  WALTER  RANSON,  M.  D.,  Ph.  D. 

Professor  of  Anatomy  in  Northwestern  University  Medical  School,  Chicago 


WITH    260    ILLUSTRATIONS 
SOME  OF  THEM  IN  COLORS 


PHILADELPHIA  AND  LONDON 

W.    B.   SAUNDERS  COMPANY 

1920 


Copyright,  1920,  by  \V.  B.  Saunders  Company 


(1 


PRINTED    IN    AMERICA 

PRE88   OF 

W.     B.     SAUNDERS    COMPANY 

PHILADELPHIA 


PREFACE 


In  the  pages  which  follow  the  anatomy  of  the  nervous  system  has  Itch  pre- 
sented from  the  dynamic  rather  than  the  static  point  of  view;  that  is  to  say, 

emphasis  has  been  laid  on  the  developmental  and  functional  significance  of  >truc- 
ture.  The  student  is  led  at  the  very  beginning  of  his  neurologic  studies  to  think 
of  the  nervous  system  in  its  relation  to  the  rest  of  the  living  organism.  Struc- 
tural details,  which  when  considered  by  themselves  are  dull  and  tiresome,  become 
interesting  when  their  functional  significance  is  made  obvious.  This  method  of 
presentation  makes  more  easy  the  correlation  of  the  various  neurologic  courses 
in  the  medical  curriculum.  For  physiologic  and  clinical  neurology  a  knowledge 
of  conduction  pathways  and  functional  localization  is  essential,  and  this  informa- 
tion can  best  be  acquired  in  connection  with  the  course  in  anatomic  neurology. 
In  selecting  the  material  to  be  included  in  this  book  the  needs  of  the  medical 
student  have  been  kept  constantly  in  mind,  and  emphasis  has  been  placed  on 
those  phases  of  the  subject  which  the  student  is  most  likely  to  find  of  value  to 
him  in  his  subsequent  work. 

In  many  laboratories  the  head  of  the  shark  and  the  brain  of  the  sheep  have 
been  used  to  supplement  human  material.  The  book  has  been  so  arranged  as  to 
facilitate  such  comparative  studies  without  making  it  any  the  less  well  adapted 
to  courses  where  only  human  material  is  used. 

During  the  past  twenty  years  very  considerable  additions  have  been  made  to 
the  science  of  neurology,  and  the  more  important  of  these  have  been  included  in 
the  text.  While  a  detailed  presentation  of  the  evidence  concerning  new  or  dis- 
puted points  would  be  out  of  place  in  a  book  of  this  kind,  whenever  the  state- 
ments made  here  differ  from  those  found  in  other  texts  the  authority  has  always 
been  cited,  the  author's  name  and  the  date  of  his  contribution  being  given  in 
parentheses.  A  full  list  of  these  references  to  the  literature  has  been  included  in 
a  Bibliography  at  the  end  of  the  volume. 

The  terminology  adopted  is  that  of  the  B.  X.  A.,  which  has  been  used,  for 
the  most  part,  in  its  English  form.    But  in  the  case  of  the  fiber  tracts  the  Ba>le 


1 2  PREFACE 

terms  arc  often  misleading,  and  wherever  this  is  the  case,  other  names  have  been 
substituted. 

An  outline  for  a  laboratory  course  in  neuro-anatomy  has  been  included,  and 
this  has  been  so  arranged  as  to  be  easily  adapted  by  the  instructor  to  his  par- 
ticular needs. 

Free  use  has  been  made  of  material  gathered  and  arranged  by  others  in  the 
various  handbooks,  texts,  and  atlases  that  deal  with  the  nervous  system.  The 
classification  of  the  afferent  paths  and  centers  adopted  here  is  based  on  the 
work  of  Sherrington.  The  terms  which  he  introduced  and  which  are  now  coming 
into  general  use  have  been  employed.  In  the  analysis  of  the  cranial  nerves  the 
American  conception  of  nerve  components,  so  ably  presented  by  Herrick,  has 
been  utilized. 

Illustrations  have  been  borrowed  from  many  sources,  in  each  case  duly 
accredited,  and  our  indebtedness  for  permission  to  use  them  is  gladly  acknowl- 
edged. The  majority  of  the  figures  have  been  made  from  drawings  prepared  for 
this  purpose  by  Miss  M.  E.  Bakehouse.  The  large  number  of  illustrations  and 
the  excellent  manner  in  which  they  have  been  reproduced  is  to  be  credited  to  the 
generous  policy  of  the  publishers,  W.  B.  Saunders  Co.  My  thanks  are  due  to 
Dr.  Olaf  Larsell  for  reading  the  manuscript  and  for  many  valuable  suggestions, 
and  to  Mr.  Michael  Mason  for  assistance  in  reading  the  proof. 

S.  W.  Ranson. 

Chicago,  III., 
November,  1920. 


CONTENTS 


CHAPTER  I 

Origin  and  Fi  n<  noN  of  mi    Nervous  System 17 

The  Diffuse  Nervous  System  of  ( loelenterates V) 

rhe  Central  Nervous  System 20 

CHAPTER   II 

The  Neural  Tube  and  its  Derivatives 24 

The  Brain  of  the  Dogfish 26 

I  development  of  the  Neural  Tube  in  the  Human  Embryo ^1 

CHAPTER  III 

HlSTOGl  NESIS   OF   THE    NERVOUS   SYSTEM J7 

I  Jevelopmenl  of  the  Neuron 37 

Development  of  the  Spinal  Nerves 40 

Differentiation  <>f  the  Spinal  Cord 42 

CHAPTER  IV 

Neurons  and  Neuron-chains 43 

Form  and  Structure  of  Neurons 43 

Interrelation  of  Neurons 49 

The  Neuron  as  a  Trophic  Unit 51 

The  Neuron  Concept 52 

Neuron  Chains 53 

CHAPTER  V 

The  Spinal  Nerves 56 

Metamerism 58 

Functional  Classification  of  Nerve-fibers 60 

The  Spinal  Ganglia 62 

Somatic  Sensory  Fibers  and  Nerve  Endings 66 

CHAPTER  VI 

The  Spinal  Cord 73 

External  Form  and  Topography 73 

The  Spinal  Cord  in  Section 78 

Microscopic  Anatomy 85 

The  Spinal  Reflex  Mechanism 91 

CHAPTER  VII 

Fiber  Tracts  of  the  Spinal  Cord 95 

Intramedullary  Course  of  the  Dorsal  Root  Fibers 95 

Afferent  Paths  in  the  Spinal  Cord 98 

Ascending  and  Descending  Degeneration  in  the  Spinal  Cord 105 

Long  Descending  Tracts  of  the  Spinal  Cord 108 

CHAPTER  VIII 

General  Topography  of  the  Brain 113 

Anatomy  of  the  Medulla  Oblongata 118 

Anatomy  of  the  Pons 123 

The  Fourth  Ventricle 125 

The  Mesencephalon 129 

13 


14  CONTENTS  j 

CHAPTER  IX  Pace 

The  Structure  of  the  Medulla  Oblongata 132 

The  Rearrangement  Within  the  Medulla  Oblongata  of  the  Structures  Continued  Upward 

from  the  Spinal  Cord 133 

Decussation  of  the  Pyramids 136 

Nucleus  Gracilis,  Nucleus  Cuneatus,  and  Medial  Lemniscus 137 

Olivary  Nuclei 141 

Restiform  Body 143 

Formatio  Reticularis 144 

CHAPTER  X 

Internal  Structure  of  the  Pons 147 

Basilar  Part  of  the  Pons 147 

Dorsal  Part  of  the  Pons 149 

CHAPTER  XI 

Internal  Structure  of  the  Mesencephalon 158 

Tegmentum 158 

Basis  Pedunculi 164 

Corpora  Quadrigemina 165 

CHAPTER  XII 

The  Cranial  Nerves  and  Their  Nuclei 168 

Somatic  Efferent  Column  of  Nuclei 1 70 

Special  Visceral  Efferent  Column  of  Nuclei 1  74 

General  Visceral  Efferent  Column  of  Nuclei 177 

Visceral  Afferent  Column 180 

General  Somatic  Afferent  Nuclei 182 

Special  Somatic  Afferent  Nuclei 185 

Summary  of  the  Origin  and  Composition  of  the  Cranial  Nerves 190 

CHAPTER  XIII 

The  Cerebellum 195 

Development 195 

Anatomy 196 

Morphology 199 

Nuclei  of  the  Cerebellum ' 203 

Cerebellar  Peduncles 204 

Histology  of  the  Cerebellar  Cortex 206 

Efferent  Cerebellar  Tracts 211 

CHAPTER  XIV 

The  Diencephalon  and  Optic  Nerve 213 

Thalamus 213 

Epithalamus  and  Metathalamus 220 

Hypothalamus 222 

Third  Ventricle 223 

Visual  Apparatus 225 

CHAPTER  XV 

External  Configuration  of  the  Cerebral  Hemispheres 229 

Development  of  the  Cerebral  Hemispheres 229 

The  Dorsolateral  Surface 232 

The  Medial  and  Basal  Surfaces 238 


CONTENTS  I - 

CHAPTER  XVI 

Imikwi   Configuration  oi    mi    Cerebral  Hemispheres 243 

Corpus  Callosum                                      243 

Lateral  Ventri<  lea  

Basal  <  >anglia  "I  the  Telencephalon   252 

Intern. il  ( lapsule _';7 

Connections  of  the  Corpus  Striatum  and  I  halamus 262 

(  II  ATI  ER  XVII 

'I'm    Rhine  mi  e  phali  »w 265 

Parts  Seen  on  tin-  B.i-.il  Surface  of   the  Hrain _",: 

I  [ippocampus   269 

Fornix 270 

Anterior  Commissure 11  ^ 

Strm  tun-  and  Connections  of  the  Several  Parts  of  the  Rhinencephalon 274 

Olfactory  Pathways  280 

CHAPTER  XVIII 
Thf.  Cortex  and  Medullary  Center  ok  the  Cerebral  Hemisphere 

Structure  of  the  Cerebral  Cortex 28.1 

Cortical  Areas 2X7 

Localization  of  Cortical  Functions 290 

The  Medullary  Center  of  the  Cerebral  Hemisphere 296 

CHAPTER  XIX 

The  Great  Afferent  Systems 302 

Exteroceptive  Pathways  to  the  Cerebral  Cortex 302 

Spinal  Path  for  Touch  and  Pressure 303 

Spinal  Path  for  Pain  and  Temperature  Sensations 306 

Secondary  Trigeminal  Paths 307 

Neural  Mechanism  for  Hearing 309 

Neural  Mechanism  for  Sight 310 

Proprioceptive  Pathways 311 

Spinal  Proprioceptive  Paths  (Muscle  Sense) 311 

Cerebellar  Connections  of  Vestibular  Nerve 314 

CHAPTER  XX 

Efferent  Paths  and  Reflex  Arcs 316 

The  Great  Motor  Path 517 

The  Cortico-ponto-cerebellar  Path 325 

The  Cerebello-rubro-spinal  Path 326 

Important  Reflex  Arcs 327 

CHAPTER  XXI 

The  Sympathetic  Nervous  System 

Fundamental  Facts  Concerning  Visceral  Innervation 

Structure  of  the  Sympathetic  Ganglia 541 

Composition  of  Sympathetic  Nerves  and  Plexuses J45 

Architecture  of  the  Sympathetic  Nervous  System 346 

Important  Conduction  Paths  Belonging  to  the  Autonomic  Nervous  System 352 

A  Laboratory  Outline  of  Neuroanatomy 

Bibliography 


Index 383 


THE    ANATOMY    OF    THE    NERVOUS    SYSTEM 
FROM   THE   STANDPOINT    OF    DEVELOP- 
MENT AND   FUNCTION 


CHAPTER  I 


THE  ORIGIN  AND  FUNCTION  OF  THE  NERVOUS  SYSTEM 

Irritability  and  conductivity,  which,  as  every  biological  student  knows, 
are  two  of  the  fundamental  properties  of  protoplasm,  reach  their  maximum 
development  in  the  highly  differentiated  tissue  of  the  nervous  system.  Indeed, 
it  is  in  response  to  the  need  for  increased  sensitiveness  to  stimuli  and  for  better 
transmission  of  the  impulses  aroused  by  them  that  the  nervous  system  has 
developed  and  been  perfected  in,  the  long  process  of  evolution  which  has  cul- 
minated in  man. 

When  an  ameba  is  touched  with  a  pointed  glass  rod  it  moves  away  from 
the  source  of  stimulation.  Changes  are  initiated  in  the  superficial  protoplasm 
which  are  transmitted  through  the  unicellular  organism,  resulting  in  a  flowing 
out  of  pseudopodia  on  the  opposite  side.  Through  a  continuation  of  this  stream- 
ing motion  the  entire  organism  moves  forward.  Thus  the  relatively  undif- 
ferentiated living  substance  of  which  it  is  composed  receives  the  stimulus, 
transmits  the  resulting  disturbance,  and  carries  out  the  appropriate  response. 

When  in  the  place  of  unicellular  organisms  we  stud}'  simple  metazoa,  the 
sea-anemones  for  example,  we  find  that  considerable  differentiation  has  occurred 
among  the  component  cells.  A  cuticle  has  formed,  designed  to  protect  the 
subjacent  parts  from  the  action  of  the  surrounding  objects,  while  other  cells 
have  differentiated  in  the  direction  of  contractile  elements  or  muscle  cells. 
Because  the  general  body  surface  has  been  adapted  to  cope  with  the  environ- 
ment it  becomes  necessary  to  have  certain  cells  at  the  surface  which  are  sensi- 
tive to  environmental  changes.  These  sensory  elements  are  able  to  transmit 
the  waves  of  activation  developed  in  them  directly  to  the  subjacent  muscle 
cells.     But  in  higher  animals,  because  of  the  large  size  of  the  body  and  the 

2  17 


lb  THE    NERVOUS    SYSTEM 

complicated  reactions  required,  long  lines  of  communication  have  been  estab- 
lished between  peripheral  sense  organs  and  muscle-fibers  in  widely  separated 
parts  of  the  body. 

The  sensor}-  elements  and  the  lines  of  communication  constitute  the  nervous 
system  and,  together  with  the  musculature,  the  neuromuscular  mechanism. 
It  is  well  to  keep  in  mind  the  fact  that  the  nervous  system  was  developed  for  the 
purpose  of  enabling  the  musculature  to  react  to  changes  in  the  environment  of 
the  organism.  But  in  all  higher  animals  the  nervous  system  responds  not  only 
to  stimuli  from  without  but  also  to  stimuli  from  within  the  body,  and  helps  to 


in 


Fig.  1. — Stages  in  the  differentiation  of  the  neuromuscular  mechanism:  A  to  C,  Hypothetic 
early  stages:  A,  epithelial  stage;  B,  muscle  cell  at  the  stage  of  the  sponge;  C,  partially  differen- 
tiated nerve-cell  in  proximity  to  fully  differentiated  muscle-cell;  D,  nerve-  and  muscle-cell  of 
coelenterate  stage;  E,  a  type  of  receptor-effector  system  found  in  many  parts  of  sea-anemones,  in- 
cluding not  only  receptors,  r,  with  their  nerve-nets,  and  of  muscle  cells,  m,  but  also  of  ganglion 
cells,  g,  in  the  nerve-net;  F,  section  at  right  angles  to  the  sphincter  of  the  bell  of  a  jellyfish  (Rhizos- 
toma):    e,  epithelium  of  the  subumbrellar  surface;  n,  nervous  layer;  m,  muscle  layer.     (Parker.) 

bring  about  an  internal  adjustment  of  part  with  part.  Here  again  it  acts  as  a 
sensitive  mechanism  for  receiving  stimuli  and  conducting  them  to  the  appro- 
priate organs  of  response.  These  organs  through  which  the  nervous  system 
produces  its  effects  are  known  as  effectors.  While  muscles  and  glands  are  by 
far  the  most  important  effectors,  we  must  also  include  certain  pigmented  cells 
u>r  chromatophores)  and  electric  and  phosporescent  organs  under  this  heading 
Except  for  the  reactions  produced  through  such  effectors  the  nervous  system 
would  be  meaningless. 

We  can  best  understand  the  significance  of  the  nervous  system  if  we  trace 
its  early  history.     This,  as  it  has  been  interpreted  by  Parker  (1919),  makes  an 


lilt:    ORIGIN     \M»    FUNCTION    OF    mi     NERVOUS   SYSTEM 


M, 


interesting  story.  According  to  this  author  contractile  tissue  develops  before 
any  trace  oi  the  nervous  system  appears.  In  sponges,  which  arc  devoid  of 
nervous  elements,  the  oscula  open  and  close  in  response  to  appropriate  timuli. 
These  movements  are  brought  aboul  by  a  contractile  tissue  no1  unlike  mooth 
muscle.  The  active  element  or  effector  is  thus  the  first  to  make  it-  appearance 
and  at  this  stage  is  brought  into  action  by  direct  stimulation.  Nexl  in  the  order 
of  development  is  the  sensory  cell,  derived  from  the  epithelium  in  the  neigh- 
borhood of  an  effector,  and  specially  differentiated  to  receive  stimuli  and  trans 
mit  them  to  the  underlying  muscle  (Fig.  1,  I)).  This  stage  of  development  i- 
reached  by  such  ccelenterates  as  the  sea-anemones.  The  advantage  which 
these  forms  derive  from  the  specialized  sensory  cells  or  receptors  is  -ten  in  the 
character  of  their  responses,  which  arc  more  rapid  than  those  of  sponges.     Such 


Cerebral  ganglion  v 


Esophageal  connective 
Pharynx 

Ventral  nerve  cord 


Cerebral  ganglion 
Pharynx 

Esophageal  connective 

Ventral  nerve  cord 


A  B 

Fig.  2. — Anterior  portion  of  the  nervous  system  of  the  earthworm:  A,  Lateral  view;  B,  dorsal  view. 

a  sensory  cell  may  be  compared  to  a  percussion  cap  through  which  a  charge  of 
powder  is  ignited. 

But  ccelenterates  usually  present  a  more  complex  arrangement  of  receptor 
and  effector  elements  than  that  indicated  in  Fig.  1 ,  D.  Fine  branches  from  the 
sensory  cells  anastomose  with  each  other  and  form  a  nervous  net  within  which 
are  scattered  nerve-cells.  Such  a  nerve  net  is  seen  in  many  parts  of  sea-ane- 
mones (Fig  1,  E)  and  is  well  developed  in  the  jellyfish  (Fig.  1,  F).  It  seems 
capable  of  conveying  nerve  impulses  coming  from  the  sensory  cells  in  all  direc- 
tions through  the  bell-shaped  body  of  the  jellyfish  and  to  muscle-fibers  far  dis- 
tant from  the  receptors  involved.  The  conduction  of  nerve  impulses  from 
receptors  to  effectors  seems  to  occur  diffusely  through  the  net — not  in  stated 
directions  nor  along  fixed  paths.  In  this  respect  the  diffuse  nervous  system  of 
the  ccelenterates  is  in  contrast  with  the  more  centralized  system  in  the  worms. 


20 


THE    NERVOUS    SYSTEM 


The  sensory  cells  are  not  so  directly  connected  with  muscle-fibers  in  the 
worms  as  in  the  sea-anemones,  for  between  receptor  and  effector  there  is  here 
interposed  a  central  nervous  system.  This  system,  as  it  appears  in  the  earth- 
worm, is  illustrated  in  Fig.  2.  It  consists  of  a  cerebral  ganglion  dorsal  to  the 
buccal  cavity  and  a  row  of  ventrally  placed  ganglia  bound  together  by  a  ventral 
nerve  cord.  The  most  anterior  of  the  ventral  series  of  ganglia  is  connected  to 
the  dorsal  one  by  nerve  strands  on  either  side  of  the  esophagus.  The  ganglia 
of  the  ventral  cord  are  placed  so  that  one  occurs  in  each  body  segment,  and 
from  each  three  pairs  of  nerves  run  to  the  skin  and  muscles  of  that  segment. 
The  arrangement  of  the  constituent  elements  can  best  be  studied  in  transverse 
sections  (Fig.  3).  The  sensory  cells  are  located  in  the  skin,  and  from  each  of 
them  a  fiber  runs  along  one  of  the  nerves  into  the  ganglion,  within  which  it 
branches,   helping  to  form   a   network  known  as  the  neuropil.     Within  each 


Fig.  3. — Transverse  section  of  the  ventral  chain  and  surrounding  structures  of  an  earthworm: 
cm,  Circular  muscles;  ep,  epidermis;  tin,  longitudinal  muscles;  mc,  motor  cell-body;  mf,  motor 
nerve-fiber;  sc,  sensory  cell-body;  sf,  sensory  nerve-fiber;  vg,  ventral  ganglion.     (Parker.) 

ganglion  are  found  large  nerve-cells  from  which  fibers  run  through  the  nerves 
to  the  segmental  musculature.  Here  we  have  the  necessary  parts  for  the  sim- 
plest reflex  arc.  Stimulation  of  the  sensory  cell  causes  nerve  impulses  to  travel 
through  its  fiber  to  the  neuropil,  thence  to  a  motor  cell,  and  finally  along  a  proc- 
ess of  the  latter  to  the  muscle.  In  other  words,  we  have  a  receptor,  conductor, 
center,  another  conductor,  and  finally  an  effector;  and  all  this  is  for  the  purpose 
of  bringing  the  muscle-fiber  under  the  influence  of  such  environmental  changes 
as  are  able  to  stimulate  the  sensitive  receptor. 

In  addition  to  the  primary  sensory  and  motor  elements  just  enumerated  the 
ganglia  contain  nerve-cells  the  fibers  of  which  run  from  one  ganglion  to  another 
and  serve  to  associate  these  in  co-ordinated  activity.  These  internuncial  ele- 
ments serve  to  establish  functional  connections  among  the  different  parts  of 
the  ganglionated  nerve  cord  that  constitutes  the  central   nervous  apparatus; 


Mil     ORIGIN    AND    FUNCTION    01     mi     NERVOUS    SYSTEM 

and  they  lie  entirely  within  this  central  organ.  The  slow  waves  of  contraction 
that  pass  from  bead  to  tail  as  the  worm  creeps  forward  may  be  advanced  from 
segment  to  segment  by  such  internuncial  or  association  elements. 

The  nervous  system  of  the  earthworm  differs  from  thai  of  the  ccelenterate 
in  many  ways,  but  the  fundamental  difference  is  one  of  centralization.  In  the 
former  the  greater  part  of  it  has  separated  from  the  skin  and  become  con 
centrated  in  a  series  of  interconnected  ganglia  which  serve  as  a  central  nervous 
system.  These  ganglia  receive  nerve  fibers,  coming  from  the  sense  organs,  and 
give  off  others,  going  to  the  muscles;  and  the  fibers  are  brought  together  and 
grouped  into  nerves  for  convenience  of  passage.  The  neuropil  within  a  ganglion 
offers  a  variety  of  pathways  to  each  incoming  impulse  which  may  accordingly 
find  its  way  out  along  one  or  more  of  several  motor  fibers.  The  spreading  of 
nerve  impulses  through  the  chain  of  ganglia  is  facilitated  by  the  presence  of  the 
association  fibers  already  mentioned.  Nevertheless,  conduction  is  not  diffuse 
as  in  ti.e  nerve  net  of  the  medusa,  but  occurs  along  definite  and  more  or  less 
restricted  lines.  This  is  well  illustrated  by  the  experiment  cited  by  Parker: 
"If  an  earthworm  that  is  creeping  forward  over  a  smooth  surface  is  suddenly 
cut  in  two  near  the  middle,  the  anterior  portion  will  move  onward  without  much 
disturbance,  whereas  the  posterior  part  will  wriggle  as  though  in  convulsions. 
This  reaction,  which  can  be  repeatedly  obtained  on  even  fragments  of  worms, 
shows  that  a  single  cut  involves  a  stimulation  which  in  a  posterior  direction 
gives  rise  to  a  wholly  different  form  of  response  to  what  it  does  anteriorly;  in 
other  words,  transmission  in  the  nerve  cord  of  the  worm  is  specialized  as  com- 
pared with  transmission  in  the  nervous  net  of  the  ccelenterate."  In  the  gan- 
glionated  cord  of  the  earthworm,  as  here  described,  we  find  many  of  the  features 
characteristic  of  the  central  nervous  system  of  higher  forms. 

The  vertebrate  nervous  system  has  much  in  common  with  that  of  the  earth- 
worm. The  central  nervous  system  of  the  annelid  is  split  off  from  the  ectoderm 
by  a  process  of  delamination,  as  will  be  seen  by  comparing  the  ventral  nervous 
cord  of  the  marine  worm,  Sigalion,  with  that  of  the  earthworm  (Figs.  3,  4). 
Through  a  comparable  process  of  infolding  of  the  ectoderm  to  form  a  neural 
tube  there  is  developed  the  central  nervous  system  of  the  vertebrate  (Fig.  6). 
The  dorsal  position  of  the  neural  tube  in  vertebrates  as  compared  with  the 
ventral  position  of  the  solid  nerve  cord  of  the  annelid  offers  some  difficulty  and 
has  led  to  ingenious  theories  in  explanation  of  their  phylogenetic  relationship, 
theories  which  we  need  not  consider  here  (Gaskell,  1908).  In  primitive  chor- 
dates,  such  as  the  amphioxus,  we  already  have  a  simple,  dorsally  placed,  neural 


22 


THE    NERVOUS    SYSTEM 


tube  associated  with  segmental  nerves.  In  true  vertebrates  the  anterior  end  of 
the  neural  tube  becomes  irregularly  enlarged  to  form  the  brain,  while  the  pos- 
terior end  remains  less  highly  but  more  uniformly  developed  and  forms  the 
spinal  cord. 

The  primary  motor  nerve-cells  of  vertebrates  resemble  very  closely  those  of 
invertebrates  in  being  located  within  the  central  nervous  system  and  in  send- 
ing motor  nerve-fibers  to  the  muscles  (Fig.  31).  The  primary  sensory  cells  lie 
outside  the  central  system,  as  in  invertebrates.  Those  for  smell  are  located  in 
the  olfactory  epithelium.  But  all  others  have  migrated  centrally  along  the 
sensory  fibers,  and  now  send  one  process  toward  the  periphery  and  another  into 


Fig.  4. — Transverie  section  of  the  ventral  nervous  cord  of  Sigalion:  bm,  Basement  mem- 
brane; c,  cuticula;  e,  epidermis;  gc,  ganglion-cells;  n,  nerve-fibers  and  neuropil;  s,  space  occupied 
by  vacuolated  supporting  tissue.     (Parker,  Hatschek.) 

the  central  system.  The  relative  positions  of  these  cells  in  the  annelid,  mollusc, 
and  vertebrate  are  illustrated  in  Fig.  5.  In  the  latter  the  sensory  cells  are  aggre- 
gated into  masses  known  as  the  cerebrospinal  ganglia,  which  are  associated 
with  peripheral  nerves  and  are  usually  placed  near  the  point  of  origin  of  these 
nerves  from  the  brain  or  spinal  cord.  A  comparison  of  Figs.  3  and  31  will  show 
a  striking  similarity  between  the  simple  reflex  arc  in  the  earthworm  and  in  man. 
If  space  permitted  we  might  trace  the  development  of  the  central  nervous  sys- 
tem in  some  detail,  but  perhaps  enough  has  been  given  to  suggest  that  the 
nervous  system  of  man  represents  the  culmination  of  a  long  process  of  evolu- 
tion which  began  with  a  simple  sensory  mechanism  like  that  of  the  sea-anemones. 
We  shall  be  concerned  with  a  study  of  the  vertebrate  nervous  system,  almost 


THE    ORIGIN    AND    FUNI   HON    01      Mil     NERVOUS    SYSTEM 


23 


exclusively  with  that  of  the  mammal,  and  more  particularly  with  that  of  man. 
In  man  we  are  so  accustomed  to  think  of  the  uervous  system  .1-  the  organ  and 
agent  of  the  mind  that  its  true  physiologic  position  is  often  forgotten.  In  this 
Introductor)  chapter  we  have  attempted  to  show  that  the  primary  function  of 
the  Dervous  system  is  to  receive  stimuli  arising  from  changes  in  the  environment 
or  within  the  organism,  and  to  transmit  these  to  effectors  which  bring  about 
the  adjustments  uecessary  for  life.  Biologically  speaking,  the  nervous  system 
is  not  to  he  regarded  as  an  intelligence  bureau,  which  gathers  information  for 


Fig.  5. — Peripheral  sensory  neurons  of  various  animals:  A,  Oligochaetic  worms  (Lumbricus); 
B,  polychaetic  worms  (Nereis);  C,  molluscs  (Limax);  D,  vertebrates.  The  figure  illustrates  the 
gradual  change  in  the  position  of  the  sensors-  cells  in  the  phylogenetic  series:  e,  Epithelial  cells  of 
sensory  surface;  c,  cuticula;  sz,  cell-body  of  peripheral  sensory  neuron;  rm,  rete  Malpighii  of  epi- 
dermis; sn,  axon;  co,  central  nervous  system.      (Barker,  Retzius.) 

a  sovereign  mind,  enthroned  within  the  brain,  nor  yet  as  a  chief  executive  officer 
to  carry  out  that  sovereign's  decrees.  Sensory  impulses  from  many  source.- 
reach  the  brain,  where  they  pass  back  and  forth  through  a  multitude  of  asso- 
ciation paths,  augmenting  or  inhibiting  each  other  before  they  finally  break 
through  into  motor  paths.  Previous  experience  of  the  individual,  having  left 
its  trace  in  the  organization  of  the  central  nervous  system,  alters  the  character 
of  the  present  reactions.  It  is  in  connection  with  the  neural  activity  involved 
in  these  complex  associational  processes  that  consciousness  appears — shall  I 
say  as  a  by-product? — at  least  as  a  parallel  phenomenon. 


CHAPTER  II 

THE  NEURAL  TUBE  AND  ITS  DERIVATIVES 

Infolding  of  the  Neural  Tube. —  The  vertebrate  nervous  system  develops 
from  a  thickened  plate  of  ectoderm  along  the  middorsal  line  of  the  embryo. 
By  the  infolding  of  this  neural  plate  there  is  formed  the  neural  groove,  which 
becomes  transformed  into  the  neural  tube  (Fig.  6).  The  neural  tube  detaches 
itself  from  the  superficial  ectoderm  and  gives  rise  through  a  thickening  of  its 
walls  to  the  brain  and  spinal  cord.     The  latter  is  formed  by  a  process  of  uniform 


Neural  groove         Neural  plate 


Neural  groove     Neural  plate 


Ectoderm 


Neural  groove 


Neural  tube 


Neural  tube 


D 


Neural  cavity 


Fig.  6. — Development  of  the  neural  tube  in  human  embryos  (Prentiss-Arey):  A,  An  early  embryo 
(Keibel) ;  B,  at  2  mm.  (Graf  Spee) ;  C,  at  2  mm.  |  Mall) ;  D,  at  2. 7  mm.     CKollmann). 

thickening  in  the  walls  of  the  caudal  portion  of  the  tube.     The  derivatives  of 
the  rostral  part  are  well  illustrated  in  the  accompanying  diagram  (Fig.  7). 

Brain  Vesicles. — At  an  early  stage  in  the  development  of  any  vertebrate 
embryo  the  rostral  portion  of  the  neural  tube  is  distinguished  from  the  caudal 
part  by  the  more  rapid  development  of  the  former,  its  walls  bulging  outward 
to  form  three  bulb-like  swellings  or  vesicles,  which  together  represent  the  brain, 
and  are  named  from  before  backward,  the  prosencephalon,  mesencephalon,  and 


24 


THE    NEURAL     IM;i      WD    Us    DERIVATIVES 


25 


rhombencephalon  (Fig.  7).  The  more  rostral  vesicle  becomes  subdivided  by  .1 
oonstriction  into  the  telencephalon  and  diencephalon  (Fig.  7,  B,  (>.  The  rhom- 
bencephalon is  less  sharply  subdivided  into  a  rostral  pari,  which  includes  the 
cerebellum,  and  is  known  as  the  metencephalon  t  and  a  more  caudal  portion,  the 
myelencephalon.  The  optic  nerves  and  retinae,  not  illustrated  in  the  figure, 
develop  as  paired  evaginations  from  the  prosencephalon. 

The  Cerebral  Hemispheres.     The  telencephalon  includes  a  thickened  portion 
of  the  ventrolateral  wall  loosely  designated  as  the  corpus  striatum  or.  since  there 


-,f6-*t 


Fig.  7. — Diagrams  illustrating  the  development  of  the  vertebrate  brain:  A,  First  stage,  side 
view,  the  cavity  indicated  by  dotted  line;  B,  second  stage;  C,  third  stage,  side  view  of  a  brain  with- 
out cerebral  hemispheres;  D,  the  same  in  sagittal  section;  E,  fourth  stage,  side  view  of  a  brain  with 
cerebral  hemispheres;  F,  the  same  in  sagittal  section;  G,  dorsal  view  of  the  same  with  the  cavities 
exposed  on  the  right  side.  Rkin.,  rhinoccele;  Lat.  Vent.,  lateral  ventricle;  Int.  For.,  interventricu- 
lar foramen;  Vent.  Ill,  third  ventricle;  Vent.  71',  fourth  ventricle.  /,  Prosencephalon;  /  a.  Telen- 
cephalon; /  a-r,  Rhinencephalon;  1  a-p.  Pallium;  1  a-lt,  Lamina  terminalis;  /  a-ch,  Cerebral 
hemisphere;  1  a-cs,  Corpus  striatum;  1  b,  Diencephalon;  1  b-t,  Thalamus.  2,  Mesencephalon;  2  c, 
Optic  lobes;  2  d,  Crura  cerebri,  j,  Rhombencephalon;  j  a,  Metencephalon;  3  a-c,  Cerebellum; 
3  b,  Myelencephalon. 


is  one  of  these  on  either  side,  the  corpora  striata  (Fig.  7,  D).  Another  part  of 
the  wall  is  relatively  thin  and  is  known  as  the  pallium,  while  the  part  directly 
associated  with  the  olfactory  nerve  belongs  to  the  rhinencephalon.  The  most 
important  factor  in  the  evolution  of  the  vertebrate  brain  is  the  progressive  e\ -ag- 
ination of  the  lateral  walls  of  the  telencephalon  to  form  paired  masses,  the 
cerebral  hemispheres.  In  primitive  forms  like  the  cyclostomes  only  a  part  of  the 
rhinencephalon  has  been  evaginated,  and  in  them  the  hemisphere  consists  only 
of  an  olfactorv  bulb  and  olfactory  lobe.     This  stage  of  development  is  roughly 


26  THE    NERVOUS    SYSTEM 

indicated  in  Fig.  7,  C,  D.  In  the  selachians,  as  illustrated  in  Figs.  8,  9,  10, 
and  11,  the  evagination  has  progressed  further  than  in  cyclostomes.  Still  further 
progress  in  this  direction  has  been  made  by  the  amphibians,  the  cerebral  hemi- 
spheres of  which  have  reached  about  the  stage  of  development  indicated  in  Fig. 
7,  £,  F,  G.  Here  the  entire  lateral  wall,  including  the  pallium  and  corpus 
striatum,  has  been  evaginated  in  the  formation  of  the  cerebral  hemisphere. 

The  Brain  Ventricles. — The  portions  of  the  original  cavity  of  the  neural  tube 
which  are  contained  within  the  evaginated  cerebral  hemispheres  are  known  as 
the  lateral  ventricles  (Fig.  7,  G).  These  paired  ventricles  communicate  with  the 
median  prosencephalic  cavity  by  openings  known  as  the  interventricular  foram- 
ina. This  median  cavity,  called  the  third  ventricle,  represents  for  the  most 
part  the  cavity  of  the  diencephalon,  but  its  rostral  part,  bounded  by  the  lamina 
terminalis,  belongs  to  the  telencephalon.  It  will  be  seen  by  a  study  of  the 
accompanying  diagrams  that  this  lamina  also  belongs  to  the  telencephalon  and 
represents  in  a  certain  sense  the  rostral  end  of  the  brain.  Its  position  should 
be' carefully  noted  in  each  of  the  diagrams.  The  cavity  of  the  rhombencephalon 
is  known  as  the  fourth  ventricle  and  that  of  the  mesencephalon  as  the  cerebral 
aqueduct.  The  latter  connects  the  third  and  fourth  ventricles.  It  will  help  us 
to  understand  the  morphology  of  the  vertebrate  brain  if  we  now  consider  the 
shape  and  arrangement  of  the  various  parts  of  a  simple  brain  like  that  of  the 

dogfish. 

THE   BRAIN   OF   THE   DOGFISH— SQUALUS   ACANTHIAS 

The  telencephalon  of  the  selachian  brain  is  evaginated  to  form  a  pair  of 
laterally  placed  masses,  the  cerebral  hemispheres,  and  in  this  respect  is  at  a  stage 
of  development  not  far  removed  from  that  represented  in  diagrams  E,  F,  and  G 
of  Fig.  7.  The  long  axis  of  the  brain  is  almost  straight;  and  this  freedom  from 
ventrodorsal  curvatures  makes  it  especially  easy  to  recognize  the  various  funda- 
mental divisions  already  enumerated  and  to  understand  their  relationship. 

The  medulla  oblongata,  which  together  with  the  cerebellum  forms  the  rhom- 
bencephalon, is  continuous  at  the  caudal  extremity  with  the  cylindric  spinal 
cord,  and  within  it  the  central  canal  of  the  spinal  cord  opens  out  into  the  fourth 
ventricle  (Fig.  8).  The  medulla,  which  has  somewhat  the  shape  of  a  trun- 
cated cone,  is  considerably  larger  than  the  cord,  but  decreases  in  size  as  it  is 
traced  backward  toward  their  point  of  junction.  In  the  mammal  a  conspicuous 
transverse  bundle  of  fibers,  associated  with  the  cerebellum,  is  found  on  the 
ventral  and  lateral  aspects  of  that  part  of  the  medulla  which  belongs  to  the 
metencephalon  and  is  known  as  the  pons.     But  in  the  fish  it  is  customary  to 


MM      \l  l   R  \l.    TUBE    AM)    ITS    DERTVATIVES 


27 


consider  the  medulla  oblongata  as  extending  from  the  spinal  cord  to  the  tnesen 
cephalon.     It  forms  the  ventral  and  lateral  walls  of  the  fourth  ventricle;  and 
when  the  roof  of  this  cavity  has  been  removed  these  walls  arc  seen  to  surround 
a  long  and  rather  broad  depression     the  fossa  rhomboidea  or  floor  of  the  fourth 
ventricle    which  tapers  caudally  like  the  point  of  a  pen  (Fig.  9). 

The  cerebellum  forms  an  elongated  mass  the  rostral  end  of  which  overhangs 
the  optic  lobes,  while  the  caudal  extremity  projects  over  the  medulla  oblongata 


/Nasal  capsule 


Olfactory  nrrve  N.  I 
/  Rhinocaele 


,  Lateral  ventricle 


Olfactory  bulb 

-  -  -  Nervus  terminates 
Olfactory  trait 

Cerebral  hemisphere 

Interventricular  for 

... .Epiphysis 

-*^m   _^""  ~ '    Optic  nerve_N.  II 


-Thalamus--- 


Optic  lobes M 

trochlear  nerve  N.  Ill 


k::. 


■  Cerebellum 

Lobus  linece  lateralis. 

Facial  nerve  N.  VIL 

Acoustic  nerve  N.  VIII 

Tuberculum  acusticum. — 

Medulla  oblongata- 

Glossopharyngeal  nerve  N.  IX. 

Medial  longitudinal fasc. — 

Visceral  lobe 


Telencephalon 


-Vagus  nerve  N.  X 
.--Spinal  cord- — 


*\-Tkird  ventricle 
)  Diencephalon 

Mesoceele 
V  Mesencephalon 


I VMetaccele 

/  Metencephalon 

Cerebellum 
(caudal  part) 

Rhomboid  fossa 
Myelencephalon 


Fig.  8. — The  brain  of  the  dogfish, 
Squalus  acanthias,  dorsal  view. 


Fig.  9. — The  brain  of  the  dogfish, 
Squalus  acanthias,  with  the  ventricles 
opened,  dorsal  view. 


(Fig.  8).  Its  dorsal  surface  is  grooved  by  a  pair  of  sulci  arranged  in  the  form 
of  a  cross.  It  contains  a  cavity,  a  part  of  the  original  rhombencephalic  vesicle, 
which  communicates  with  the  fourth  ventricle  proper  through  a  rather  wick 
opening  (Fig.  11).  Behind  the  cerebellum  the  fourth  ventricle  possesses  a  thin 
membranous  roof  which  was  torn  away  in  the  preparation  from  which  Fig.  8 
was  drawn. 


28 


THE   NERVOUS    SYSTEM 


Mesencephalon. — The  optic  lobes  on  the  dorsal  aspect  of  the  mesencephalon 
are  a  pair  of  rounded  masses  separated  by  a  median  sagittal  sulcus.  They 
represent  the  bulging  roof  of  the  mesencephalic  cavity  and  are  accordingly 


Cerebellum 


Optic  lobe 
I      Thalamus 

;        Cerebral  hemisphere 


Olfactory  bulb 


Vagus  nerve  N.  X       /       /  /  \\\ 

Glossopharyngeal  nerve  N.  IX  '     /  /  \*A 

Acoustic  nerve  N.  VIII  /  \ 

Abduccns  nerve  N .  VI 


Olfactory  trad 


Optic  nerve  N.  II 
\  Inferior  lobe 
Oculomotor  nerve  N.  Ill 
_  .        .     ,       ,  ,    .  ,  „      „    .,  "      '    Saccus  vasculosus 

Trigeminal  and  facial  nerves  Nn.  V,  VII  Trochlear  nerve  N.  IV 


Fig.  10. — The  brain  of  the  dogfish,  Squalus  acanthias,  lateral  view. 

spoken  of  as  the  tectum  mesencephali.  Within  this  roof  end  the  fibers  which 
come  from  the  retime  through  the  optic  nerves.  The  floor  of  the  cavity  is  formed 
by  the  ventral  part  of  the  mesencephalon.  This  appears  like  a  direct  continua- 
tion of  the  medulla  oblongata,  and  in  the  mammal  bears  the  designation  crura 


Optic  lobe 
Epiphysis  .    Mesoceele 

Cerebellum 


Cerebral  hemisphere 
Olfactory  tract ,  /\ 

Olfactory  bulb 


Paraphysis 


Metacode 


Tubcrciilum  acusticum 
■    Tela  chorioidea 

ventricle 
rral  lobe 


Preoptic  recess 

Velum  transversum 


\       Metcnccphalon  Myclcncephalon 
Saccus  vasculosus 


Mesencephalon 
Optic  chiasma    Third  ventricle 

Fig.  11. — The  brain  of  the  dogfish,  Squalus  acanthias,  medial  sagittal  section. 


cerebri.  Emerging  from  the  roof  of  the  mesencephalon  between  the  cerebellum 
and  optic  lobe  is  the  fourth  or  trochlear  nerve,  and  from  the  ventral  aspect  of 
this  division  of  the  brain  arises  the  third  or  oculomotor  nerve. 

The  Diencephalon. — The  thin  roof  of  the  diencephalon,  which  can  easily 


THE    NEURAL    Mi:i      WD    IIS    DERIVATIVES 


be  torn  away  so  as  to  expose  the  third  ventre  le  I  igs.  8,  9),  is  atta<  bed  bj  its 
caudal  margin  to  a  ridge  containing  a  pair  of  knob  like  thickenings  the  habe- 
nular  nuclei  and  a  commissure  connecting  tin-  tw<>  (Fig.  11).  From  a  point 
just  caudal  to  the  middle  of  this  commissure  there  projects  forward  over  the 
membranous  roof  oi  the  ventricle  a  slender  tube,  the  epiphysis  cerebri  or  pineal 
body,  which  comes  in  contact  with  the  roof  of  the  >kull  and  ends  in  a  slightly 
dilated  extremity.  The  epiphysis  and  hahenular  nuclei  belong  to  the  e pi  thala- 
mus. 'The  thalamus  forms  the  thick  lateral  wall  of  the  third  ventricle  and  i- 
traversed  by  the  optic  tracts  on  their  way  to  the  optic  lobes.     The  hypothalamus 


Nasal  sac 


Epiphysis 
Superior  oblique 

Trochlear  nerve 
Medio!  reel  us 
Superior  rectus 
Lateral  rectus 
Vestibule 

Spiracle 

Semicircular  canal 
Glossopharyngeal  nerve 

Vagus 

Branchial  cleft  i 


uperficial  ophthalmu   \ '.  17/ 

Olfactory  capsule 

Inferior  oblique 

Maxillary  V 
Mandibular  V 
Palatim   VII 

Spiracle 

Hyomandibular  VII 
Glossopharyngeal 

i.  Branchial  cleft 
Vagus 


Spinal  cord  Lateral  line  branch  of  vagus 

Fig.  12. — Dissection  of  the  brain  and  cranial  nerves  of  the  dogfish,  Scyllium  catulus.  The 
eye  is  shown  on  the  left  side,  but  has  been  removed  on  the  right.  (Marshall  and  Hurst,  Parker 
and  Haswell.) 


is  relatively  large  in  the  shark  and  presents,  in  addition  to  a  pair  of  laterally 
placed  oval  masses,  or  inferior  lobes,  a  thin  walled  vascular  outgrowth,  the  saccus 
vasculosus.  Closely  related  to  the  ventral  aspect  of  the  hypothalamus  is  a  gland- 
ular mass,  derived  by  a  process  of  evagination  from  the  oral  epithelium,  and 
known  as  the  hypophysis.  For  a  picture  of  this  structure  in  the  adult  dogfish 
reference  should  be  made  to  a  paper  on  the  subject  by  Baumgartner  (1915 
On  the  ventral  surface  of  the  hypothalamus  the  optic  nerves  meet  and  cross  in 
the  optic  chiasma. 

The  telencephalon  includes  all  of  the  brain  in  front  of  the  velum  transversum, 


3° 


THE   NERVOUS    SYSTEM 


a  transverse  fold  projecting  into  the  third  ventricle  from  the  membranous  roof 
(Fig.  11),  and  consists  of  a  median  unpaired  portion,  and  of  the  two  cerebral 
hemispheres  with  their  olfactory  bulbs.  The  hemispheres  are  the  evaginated 
portions  of  the  telencephalon  and  are  partially  separated  from  each  other  by  a 


r.  ophthal.  superfic.  V 
r.  ophthal.  superfic.  VII 

n.  terminalis 

,r.  ophthal.  profundus  V 
\ Optic  nerve  (n.  II) 


r.  maxillaris  V 
r.  mandib.  V 

Supra-orbital  trunk 

Infra-orbital  trunk 
Ganglion  V 
r.  palatinus  VII 
Gang,  geniculi  VII 
Gang,  later.  VII 
r.  prespirac.  VII 


-Spiracle 

-r.  hyomandib.  VII 

n.  IX 
n.  X 

r.  lateralis  X 

r.  branchialis  X 
r.  intestinalis  X 


Fig.  13. — Diagram  of  the  brain  and  sensory  nerves  of  the  smooth  dogfish,  Mustelus  canis, 
from  above.  Natural  size.  The  Roman  numerals  refer  to  the  cranial  nerves  The  olfactory 
part  of  the  brain  is  dotted,  the  visual  centers  are  shaded  with  oblique  cross-hatching,  the  acoustico- 
lateral  centers  with  horizontal  lines,  the  visceral  sensory  area  with  vertical  lines,  and  the  general 
cutaneous  area  is  left  unshaded.  On  the  right  side  the  lateral  line  nerves  are  drawn  in  black,  the 
other  nerves  are  unshaded.      (From  Herrick's  Introduction  to  Neurology.) 

median  sagittal  fissure,  which  has  been  to  a  large  extent  obliterated  by  the 
fusion  of  their  median  walls.  The  shape  of  the  lateral  ventricle  and  the  position 
of  the  interventricular  foramina  are  shown  in  Fig.  9.  From  the  lateral  side  of 
the  rostral  end  of  the  hemisphere  there  projects  forward  the  long  and  slender 
olfactory  tract  with  a  terminal  enlargement,  the  olfactory  bulb.     This  lies  in 


I  III      NEURAL    l  l  BE     WD    I  Is    i  >  I  i ;  I  \  STIVES 


contacl  with  the  nasal  sac  to  which  it  gives  off  a  number  of  fine  nerve  bundles, 
which  together  constitute  the  olfactory  or  first  cranial  nerve.  At  the  rostral  end 
of  the  brain  an  additional  nerve  makes  its  exit  from  the  hemisphere.  Ii  is 
known  as  the  nervus  terminalis  and  can  be  Followed  forward  over  the  olfactory 

tract  and  bulb  to  the  nasal  sac  (Fig.  8). 

The  roof  of  the  selachian  forebrain  presents  a  number  of  st  ructures  of  great  morphologic 
interest,  two  of  which  have  already  been  mentioned,  namely,  the  epiphysis  and  velum 
transversum.  The  former  is  an  outpocketing  of  the  roof  of  the  diencephalon;  the  latter 
is  an  infolding  and  marks  the  line  of  separation  between  the  t  wo  divisions  of  the  prosenceph- 
alon. Rostral  to  the  velum  the  roof  of  the  telencephalon  is  evaginated  to  form  a  thin-walled 
sac,  the  paraphysis.  The  velum  and  paraphysis  arc  readily  identified  in  the  mammalian 
embryo,  hut  become  obscured  in  the  course  of  later  development.  The  morphology  of  this 
region  has  recently  been  studied  in  great  detail  by  a  number  of  American  investigators: 
Minot  (1901),  Johnston  (1909),  Terry  (1910),  Warren  (1911,  1917),  and  Bailey  (1916). 

A  good  idea  of  the  shape  and  connections  of  the  various  brain  ventricles  and 
of  the  relation  of  the  various  parts  of  the  brain  to  each  other  can  be  obtained 
from  a  study  of  Figs.  9  and  11.  In  Fig.  13  there  is  indicated  the  location  of  the 
principal  sensory  areas  of  the  brain  of  the  smooth  dogfish,  and  the  relation  of 
these  areas  to  the  corresponding  peripheral  nerves  is  apparent.  The  lateral 
line  components  of  the  seventh  and  tenth  cranial  nerves  are  indicated  in  black. 

DEVELOPMENT  OF  THE  NEURAL  TUBE  IN  THE  HUMAN  EMBRYO 

In  its  embryonic  development  the  nervous  system  of  man  presents  some- 
thing like  a  synopsis  of  the  early  chapters  of  its  phyletic  history.  The  neural 
groove  is  the  most  conspicuous  part  of  an  embryo  of  2.4  mm.  (Fig.  14).  Xear 
the  middle  of  the  body  it  has  closed  to  form  the  neural  tube,  and  from  this 
region  the  closure  proceeds  in  both  directions.  The  last  points  to  close  are 
situated  at  either  end  and  are  known  as  the  neuropores.  The  rostral  end  of  the 
groove  shows  enlargements  which  upon  closure  will  form  the  brain  vesicles. 
The  longer  portion,  caudal  to  these  enlargements,  represents  the  future  spinal 
cord.  Except  that  it  is  flexed  on  itself,  the  brain  of  the  human  embryo  of  five 
weeks  (Fig.  15)  shows  a  marked  resemblance  to  the  diagram  of  a  vertebrate 
brain  without  cerebral  hemispheres  (Fig.  7,  C,  D).  The  prosencephala  vesicle 
is  divided  by  a  constriction  into  the  telencephalon  and  diencephalon  with  freely 
intercommunicating  cavities.  The  mesencephalon  is  well  denned  and  presents 
a  sharp  bend,  the  cephalic  flexure.  The  rhombencephalon  shows  signs  of  sepa- 
ration into  the  metencephalon  and  myelencephalon  and  is  slightly  bent  dorsally 
at  the  pontine  flexure.     Another  curvature  which  develops  at  the  junction  of 


32 


THE    NERVOUS    SYSTEM 


the  brain  and  spinal  cord  is  known  as  the  cervical  flexure  (Fig.  16).  From 
the  walls  of  the  prosencephalon  there  develop  outpocke tings  on  either  side, 
which  form  the  optic  cups  and  which  are  connected  to  the  brain  by  the  optic 
stalks.  From  the  cup  develops  the  retina  and  through  the  stalk  grow  the 
fibers  of  the  optic  nerve.  These  structures  are,  therefore,  genetically  parts  of 
the  brain. 

The  Telencephalon  of  the  Human  Embryo. — By  the  time  the  embryo  has 
reached  a  length  of  13  mm.  the  brain  has  passed  into  the  stage  represented  by 


Mesencephalon 

Rhombencephalon 
Myelenccphalon 

Amnion  (cut) 


Mesodermal  segment  14 


Open  neural  groove 


Prosencephalon 


Stomodccum 


Amnion  (cut) 


Yolk  sac 


Bodv  stalk 


Fig.   14. — Human  embryo  of  2.4  mm.  showing  the  neural  tube  partially  closed.     (Kollmann.) 


diagrams  E,  F,  G  of  Fig.  7.  The  lateral  wall  of  the  telencephalon,  with  the 
corpus  striatum  and  olfactory  brain  or  rliinenccphalon,  has  been  evaginated  on 
either  side  to  form  paired  structures,  the  cerebral  hemispheres  (Fig.  16).  Ex- 
cept for  the  corpus  striatum  and  rhinencephalon  the  evaginated  wall  is  relatively 
thin,  develops  into  the  cerebral  cortex,  and  is  known  as  the  pallium.  The 
lateral  ventricles  within  the  hemispheres  represent  portions  of  the  original  telen- 
cephalic  cavity  and  communicate  with  the  third  ventricle  through  the  inter- 


llll     Ml  K  \l.    I  l  BE     \M)    [TS    Dl. kl\  ATIVES 


33 


ventricular   foramina,  which  at  this  stage  arc  relatively   large.    The  lamina 
terminalis,  connecting  the  two  hemispheres  in  front  of  the  third  ventri<  le,  repre- 


Dietu  ephalon 


Pallium 


ephalon 


Cephalic 
flexure 


B 

Thalamus 


Pallium 


Optic 
cup 
Pontine  flexure 


liyelencephalon — J    .:fg 


Meten- 
i  ephalon 
Corpus  striatum 
Optic  recess 

Hypothalamus 


Mesencephalon 


Isthmus 


Cerebellum 


Medulla  obi 


Fig.  15. — Reconstructions  of  the  brain  of  a  7  mm.  embryo:  A,  Lateral  view;  B,  in  median  sagittal 

section.     (His,  Prentiss-Arey.) 

sents  in  a  certain  sense  the  rostral  end  of  the  brain.     Immediate]}-  behind  this 
lamina  is  a  portion  of  the  telencephalic  cavity  which  forms  the  anterior  part  of 

Cerebral  aqueduct 
Cerebral  peduncle ,      _..__■_..,,  Mesence  bhalon 

Hypothalamus    .v4j»5*^^^hfck- 
Epithalamus  ■    _Vjf\^J|  Rife, Mombenccphalic  isthmus 

Thalamus. 
Diencephalon- 


Pallium 
Telencephalon-.. 


Cerebellum 
Metcnccphalon 
Rhomboid  fossa 


Myelencephalon 


Rhincnccphalon      |    Corpus  striatum    Pons 
Lamina  terminalis 


Spinal  cord 


Fig.  16. — A  median  section  of  the  brain  of  a  13.6  mm.  human  embryo:    1,  Optic  recess;  2,  ridge 
formed  by  optic  chiasma;  3,  optic  chiasma;  4,  infundibular  recess.     (His,  Sobotta.) 


the  third  ventricle.     The  further  development  of  these  structures  is  readily 
traced  in  Fig.  17,  which  represents  the  brain  of  a  human  fetus  of  the  third 


34 


THE   NERVOUS   SYS1  I  M 


month.     The  most  striking  feature  of  the  brain  at  this  stage  is  the  great  size 
attained  by  the  cerebral  hemispheres. 

The   Diencephalon. — The   three  principal   divisions  of   the   diencephalon— 
the  thalamus,  epithalamus,  and  hypothalamus — faintly  indicated  in  an  embryo 


Thalamus 
i       Pineal  body  {epithalamus) 


Diencephalon    : 
Chorioid  plexus 

Corpus  striatum 
Telencephalon .;' 


Cerebral  peduncle 
!  Cerebral  aqueduct 
Mesencephalon 


Isthmus 
-  Cerebellum 
'  Metcnccphalon 
Rhomboid  fossa 
Myclcncephalon 


.  'tic    Hypo- 
/chiasma  physis   Medulla 

Lamina  terminalis  /  ^Hyp^ihaiamus  oblon^ata 
Rhincnccphalon 


-  Spinal  cord 
Central  canal 


Fig.  17. — The  brain  of  a  fetus  of  the  third  month  in  median  sagittal  section.     (His,  Sobotta.) 

of  13.6  mm.,  are  well  defined  by  the  third  month  (Fig.  17).  In  transverse 
sections  this  division  of  the  embryonic  brain  is  seen  to  be  composed  of  a  pair  of 
plates  on  either  side,  which  with  a  roof  and  floor  form  the  walls  of  the  ventricle 


Roof  plate  (with  chorioid  plexus)  ■ 

^j^\.Alar  plate  or  Thalamus 
Sulcus  limit ans 
Basal  plate  or  Hypothalamus 


Mammillary  recess 


Fig.  18. — Transverse  section  through  the  diencephalon  of  a  13.8  mm.  embryo.     (His,  Prentiss- 

Arey.) 

(Fig.  18).  The  dorsal  lamina  is  known  as  the  alar  plate,  the  ventral  as  the  basal 
plate.  On  either  side  these  meet  at  an  angle,  forming  the  sulcus  limitans.  These 
laminae  and  the  sulcus  limitans  between  them  can  be  traced  back  through  the 


I  111      Ml  RAL    I  1  BE    AND    I  rS    DERIVA1  1\  ES 


35 


mesencephalon  and  rhombencephalon  into  the  spinal  cord.    The  thalamus  is 
produced  by  a  thickening  in  the  alar  lamina  and  i>  separated  from  the  hypo- 
thalamus by  the  sulcus  limitans,  which  can  be  traced  as  far  as  the  optic  re 
rostral  to  the  ridge  produced  by  the  optic  chiasma. 

The  hypothalamus1  represents  the  basal  lamina  and  gives  rise  to  the  tuber 
Ctnereum,  posterior  lobe  of  the  hypophysis,  and  the  mammillary  bodies.  From  the 
dorsal  edge  of  the  alar  lamina.  \\  here  i his  is  attached  t<>  the  thin  roof  plate,  there 
is  developed  a  thickened  ridge,  the  epithalamus,  which  is  transformed  into  the 
habenula  and  the  pineal  body.  The  root"  plate  of  the  diencephalon  remains 
thin  and  forms  the  epithelial  lining  of  the  tela  chorioidea  or  roof  of  the  third 
ventricle. 

The  Mesencephalon. — The  basal  plate  of  the  mesencephalon  thickens  to 
form  the  cerebral  peduncles  (Fig.  17),  the  alar  plate  forms  the  lamina  quad- 
rigemina  in  which  are  differentiated  the  quadrigeminal  bodies;  the  cavity  be- 
comes the  cerebral  aqueduct. 

Table  Showing  Subdivisions  of  the  Neural  Tube  and  Their  Derivatives  (Modified  from  a 
Table  in  Keibel  and  Mall,  Hitman  Embryology). 


Primary  vehicles. 

Subdivisions. 

Derivatives. 

Lumen. 

f 

Prosencephalon. ... 

Telencephalon 

Cerebral  cortex, 
Corpora  striata, 
Rhinencephalon, 
Pars-optica  hypo- 
thalami. 

Lateral  ventricles. 
Rostral     portion     of 
the  third  ventricle. 

Brain 1 

Diencephalon 

Epithalamus, 

Thalamus, 

Hypothalamus, 
Hypophysis, 
Tuber  cinereum, 
Mammillary  bodies, 

Metathalamus. 

The  greater  part  of 
the  third  ventricle. 

Mesencephalon 

f 
Mesencephalon < 

Corpora  quadri- 

gemina, 
Crura  cerebri. 

Cerebral  aqueduct. 

Rhombencephalon . 

Metencephalon         < 
Myelencephalon 

Cerebellum, 

Pons, 

Medulla  oblongata.] 

Fourth  ventricle. 

Spinal  cord 

Spinal  cord. 

Central  canal. 

xThe  pars  optica  hypothalami,  including  the  optic  chiasm,  is,  properly  speaking,  not  a  part 
of  the  hypothalamus  at  all,  but  belongs  to  the  telencephalon  (Johnston,  1909,  Jour.  Comp.  ^eitr  , 
vol.  19,  and  1912,  Jour.  Comp.  Neur.,  vol.  22). 


36  THE    NERVOUS    SYSTEM 

The  Rhombencephalon. — The  ventral  part  of  the  rhombencephalon,  includ- 
ing both  alar  and  basal  plates,  thickens  to  form  the  pons  and  medulla  oblongata 
(Fig.  17).  Most  of  the  roof  of  this  division  remains  thin  and  forms  the  epithelial 
lining  of  the  tela  chorioidea  of  the  fourth  ventricle.  But  in  the  caudal  portion 
of  the  myelencephalon  the  lumen  of  the  neural  tube  becomes  completely  sur- 
rounded by  thickened  walls,  forming  the  central  canal  of  the  closed  portion  of 
the  medulla.  The  posterior  edge  of  the  alar  plate  in  the  metencephalon  becomes 
greatly  thickened  and.  fusing  across  the  median  line  with  the  similar  structure 
of  the  opposite  side,  forms  the  anlage  of  the  cerebellum  (Figs.  17.  137).  Later 
we  shall  see  that,  in  general,  motor  structures  develop  from  the  basal,  and  sen- 
sory parts  from  the  alar,  plate. 

The  table  on  page  35  gives  in  brief  the  principal  derivatives  of  the 
neural  tube. 


CHAPTER  III 

HISTOGENESIS  OF  THE  NERVOUS  SYSTEM 

Early  Stages  in  the  Differentiation  of  the  Neural  Tube.  Hardesty  (1904) 
has  given  a  good  account  of  the  early  development  of  the-  spinal  cord  in  the  pig. 
At  first  the  neural  plate  consists  of  a  single  layer  of  ectodermal  cells  I  Fig.  19,  A). 
These  proliferate  and  lose  their  cell  boundaries.  When  the  neural  tube  has 
closed  its  wall  is  formed  of  several  layers  of  fused  cells — a  syncytium — bounded 
by  an  external  and  an  internal  limiting  membrane  (Fig.  19,  B,  C).  The  syn- 
cytium now  becomes  more  open  and  sponge-like  in  structure.  The  nuclei  are 
so  arranged  that  three  layers  may  be  differentiated:  (1)  an  ependymal  layer, 
(2)  a  mantle  layer,  with  man)-  nuclei,  and  (3)  a  marginal  or  non-nuclear  layer. 
The  ependymal  layer  is  represented  by  a  row  of  elongated  nuclei,  among  which 
are  found  the  large  mitotic  nuclei  of  the  germinal  cells. 

These  germinal  cells  divide  and  give  rise  to  ependymal  cells,  and  to  the  indif- 
ferent cells  of  the  mantle  layer.  Through  division  of  the  latter  spongioblasts 
and  neuroblasts  are  formed.  From  the  former  comes  the  neuroglia  or  supporting 
tissue  of  the  nervous  system,  while  from  the  latter  are  derived  the  nerve-cells 
and  fibers. 

The  Development  of  the  Neuron. — A  neuron  may  be  defined  as  a  nerve- 
cell  with  all  its  processes;  and  each  is  derived  from  a  single  neuroblast.  From 
the  pear-shaped  neuroblast  a  single  primary  process  grows  out.  and  this  be- 
comes the  axis-cylinder  of  a  nerve-fiber  (Fig.  20).  Other  processes  which  de- 
velop later  become  the  dendrites.  The  primary  process,  or  axon,  grows  into  the 
marginal  layer,  within  which  it  may  turn  and  run  parallel  to  the  long  axis  of  the 
neural  tube  as  an  association  fiber;  or  it  may  run  out  of  the  neural  tube  in  a  ven- 
trolateral direction  as  a  motor  axon.  In  this  way  the  motor  fibers  of  the  cere- 
brospinal nerves  are  laid  down.  The  axis-cylinder  of  each  represents  a  process 
which  has  grown  out  from  a  neuroblast  in  the  basal  plate  of  the  neural  tube. 

Development  of  Afferent  Neurons. — The  sensory  or  afferent  libers  of  the 
spinal  nerves  take  origin  from  neuroblasts  which  are  from  the  beginning  out- 
side the  neural  tube.  These  neuroblasts  are  derived  from  the  neural  crest,  a 
longitudinal  ridge  of  ectodermal  cells  at  the  margin  of  the  neural  groove,  where 
this  becomes  continuous  with  the  superficial  ectoderm.     At  first  in  contact  with 

37 


38 


THE    NERVOUS    SYSTEM 


the  dorsal  surface  of  the  neural  tube,  the  neural  crest  soon  separates  from  it 
and  comes  to  lie  in  the  angle  between  it  and  the  myotomes.  In  this  position 
the  neural  crest  gives  rise  to  a  series  of  sensory  ganglia.  From  neuroblasts 
located  in  these  ganglia  arise  the  sensory  fibers  of  the  cerebrospinal  nerves. 


Marginal  layer        Manlle  layer        Ependymal  layer 


Germinal 

ell 


Marginal  layer     Ependymal  layer 
Mesoderm        Marginal  layer 


5"  ..SfSfV--*,; 

***£»*?    -V.' 


Internal  limiting  membrane 

Ependymal  layer 


Mm 

'V 


£4  ^Germinal 


i 


ce// 


SpHfe 


External  limiting  membrane 
External  limiting  membrane 


Mantle  layer        Internal  limiting  membrane 

Germinal  cell        Internal  limiting  membrane 


Mesoderm    Marginal  layer         *  Mantle  layer  Ependymal  layer 

Fig.  19. — Early  stages  in  the  differentiation  of  the  neural  tube:  A,  From  a  rabbit  embryo 
before  closure  of  neural  tube;  B,  from  a  5  mm.  pig  embryo  after  closure  of  tube;  C,  from  a  7  mm. 
pig  embryo;  D,  from  a  10  mm.  pig  embryo.  *,  Boundary  between  nuclear  and  marginal  layers. 
(Hardesty,  Prentiss-Arey.) 


This  last  statement  requires  some  qualification.  The  fibers  of  the  olfactory  nerve 
arise  from  cells  in  the  olfactory  mucous  membrane.  The  fibers  of  the  mesencephalic  root 
of  the  trigeminal  nerve,  which  in  all  probability  are  sensory,  arise  from  cells  located  within 
the  mesencephalon.  The  optic  nerve  is  also  an  exception,  but  this  is  morphologically  a 
fiber  tract  of  the  brain  and  not  a  true  nerve.  An  ingenious  theory,  advanced  by  Schulte  and 
Tilney  (1915),  attempts  to  bring  this  mesencephalic  root  and  the  optic  nerve  into  more  ob- 


HISTOGENESIS    OF    THE    NERVOUS    SYSTEM 


39 


vious  relation  with  the  other  sensory  nerves.     They  assume  thai  the  pari  of  the  neural  i 

which  lies  rostral  to  the  anlage  Of  the  semilunar  ganglion,  fails  tO  separate  from  the  neural 

tube.     From  this  pari  of  the  neural  crest,  retained  within  the  brain,  they  would  derive  the 
mesencephalic  nucleus  of  the  trigeminal  nerve  and  the  optic  vesicles. 

On  I  he  ot  her  hand,  there  are  observal  ions  which  tend  to  show   t  hat   some  of  t  he  cranial 
sensory  ganglia  arc  derived  at   least   in  pari   from  other  sources  than  the  neural  crest.      This 

i>  especially  true  of  the  acoustic  ganglion  (Strecter,  1912).     According  to  Landacre  (1910) 

many  of  the  sensory  ganglion  nils  ol  the  seventh,  ninth,  and  tenth  nerves  are  derived  from 


Fig.  20. — A,  Transverse  section  through  the  spinal  cord  of  a  chick  embryo  of  the  third  day 
showing  neuraxons  (F)  developing  from  neuroblasts  of  the  neural  tube  and  from  the  bipolar 
ganglion  cells,  d.  B,  Neuroblasts  from  the  spinal  cord  of  a  seventy-two-hour  chick.  The  three  to 
the  right  show  neurofibrils;  C,  incremental  cone.      (Cajal,  Prentiss-Arey.) 

thickened  patches  of  the  superficial  ectoderm,  known  as  placodes,  with  which  the  ganglia  of 
these  nerves  come  in  contact  at  an  early  stage  in  their  embryonic  development.  The 
acoustic  ganglion  of  the  eighth  nerve  seems  also  to  have  a  similar  origin,  i.  c.  from  the  cells 
of  the  otic  vesicle  which  is  formed  by  a  process  of  invagination  from  the  superficial  ectoderm. 

The  neuroblasts  of  these  ganglia  become  bipolar  through  the  development 
of  a  primary  process  at  either  end  (Fig.  21).  Originally  bipolar,  a  majority  of 
these  sensory  neurons  in  the  mammal  become  unipolar  through  the  fusion  of 
the  two  primary  processes  for  some  distance  into  a  single  main  stem.  Beyond 
the  point  of  fusion  this  divides  like  a  T  into  two  primary  branches,  one  of  which 


4Q 


THE    NERVOUS    SYSTEM 


is  directed  centrally,  the  other  peripherally.  The  centrally  directed  branch 
grows  into  the  neural  tube  as  a  sensory  root  fiber  (Fig.  20,  A,  d);  the  other  grows 
peripherally  as  an  afferent  fiber  of  a  cerebrospinal  nerve.  This  general  state- 
ment requires  some  qualification.  It  may  be  that  some  bipolar  neuroblasts 
become  unipolar  by  the  absorption  of  one  of  the  primary  processes,  while  the 
remaining  one  divides  dichotomously  into  central  and  peripheral  branches 
(Streeter,  1912).     It  should  also  be  noted  that  the  cells  of  the  sensory  ganglia 

of  the  acoustic  nerve  remain  bipolar 
throughout  life. 

Development  of  the  Spinal  Nerves. 
— We  have  traced  the  development 
of  the  chief  elements  entering  into 
the  formation  of  the  cerebrospinal 
nerves,  and  will  now  see  how  these 
are  combined  in  a  typical  spinal  nerve. 
The  spinal  ganglion,  derived  from 
the  neural  crest,  contains  bipolar 
neuroblasts,  which  are  transformed 
into  unipolar  neurons.  The  axon  of 
such  a  nerve-cell  divides  into  a  cen- 
tral branch,  running  through  the 
dorsal  root  into  the  spinal  cord,  and 
a  peripheral  branch,  running  distally 
through  the  nerve  to  reach  the  skin 
or  other  sensitive  portion  of  the  body. 
Mingled  with  these  afferent  fibers  in 


Fig.  21. — A  section  of  a  spinal  ganglion  from 
a  44  mm.  fetus,  showing  stages  in  the  trans- 
formation of  bipolar  neurons,  A,  into  unipolar 
neurons,  B.     Golgi  method.     (Cajal.) 


the  spinal  nerves  are  efferent  axons  which  have  grown  out  from  neuroblasts  in 
the  basal  plate  of  the  spinal  cord,  through  the  ventral  root,  and  are  distributed 
by  way  of  the  spinal  nerve  to  muscles. 

So  far  we  have  dealt  only  with  the  origin  of  the  axis-cylinders  of  the  nerve- 
fibers.  But  these  soon  become  surrounded  by  protective  sJicaths  which  are  also 
ectodermal  in  origin.  In  the  path  of  the  outgrowing  axons  there  are  seen  nu- 
merous spindle-shaped  ectodermal  cells,  which  have  migrated  from  the  anlage 
of  the  spinal  ganglia  (Harrison,  1906),  and  perhaps  also  from  the  neural  tube 
along  the  ventral  roots  (Held.  1909).  These  cells  form  such  a  prominent  feat- 
ure in  a  developing  nerve  that  some  workers  have  thought  the  axons  differen- 
tiate in  situ  from  them.     This  theory,  which  has  been  known  as  the  cell-chain 


HISTOGENESIS    OP    THE    NERVOUS    SYSTEM 


4  i 


hypothesis,  and  gives  to  each  axon  a  multicellular  origin,  has  been  supported  by 
Schwann,  Balfour,  Dohrn,  and  Bethe,  and  in  modified  forms  In  other  workers. 
There  are  good  reasons,  however,  for  believing  that  each  axon  arises  as  an  out- 
growth from  a  single  cell  or  neuroblast.  'I  hi-  idea,  which  i-  in  keeping  with 
what  is  known  of  the  structure  and  function  of  the  neuron  and  whi<  h  forms  an 
integral  part  of  the  now  generally  accepted  neuron  theory,  was  first  developed  in 
the  embryologic  publications  of  \\\>.  Convincing  experimental  evidence  has 
Keen  furnished  by  Harrison  (1906).  Using  amphibian  larva',  this  author  sho 
that  if  the  neural  crest  and  tube  are  removed  no  peripheral  nerves  develop. 
He   further  showed  that  isolated  nerve-cells  cultivated   in  clotted   lymph  will 


Roof  pi  ale 


Dorsal  column 


Dorsal  root 


Mantle  laxcr 


Ventral  column 


Dorsal  funiculus 


Neural  cavity 


Marginal  layer 


Ependymal  layer 

Floor  plate  Ventral  median  fissure 

Fig.  22. — Transverse  section  of  the  spinal  cord  of  a  20  mm.  human  embryo.     (Prentiss-Arey.  I 


give  rise  to  long  axons  in  the  course  of  a  few  hours.  But  the  ectodermal  cells, 
mentioned  above,  which  migrate  outward  along  the  course  of  the  developing 
nerve,  take  an  important  part  in  the  differentiation  of  the  fibers.  From  them 
is  derived  the  nucleated  sheath  or  neurilemma  of  the  peripheral  nerve-fiber. 
The  myelin  sheath  is  composed  of  a  fatty  substance  of  uncertain  origin.  It 
may  be  a  product  of  the  axon,  of  the  neurilemma,  or  of  both. 

The  sympathetic  ganglia  consist  of  cells  derived  like  those  of  the  spinal 
ganglia  from  the  neural  crest,  and.  according  to  Kuntz  (1910),  also  from  the 
neural  tube  by  migration  along  the  course  of  the  cerebrospinal  nerves.  These 
cells  become  aggregated  in  the  ganglia  of  the  sympathetic  system  and  are  asso- 
ciated with  the  innervation  of  smooth  muscle  and  glands. 


42  THE    NERVOUS    SYSTEM 

The  spinal  cord  of  a  20  mm.  human  embryo  presents  well-defined  ependymal, 
marginal,  and  mantle  layers.  Figure  22  should  be  compared  with  the  appear- 
ance presented  by  a  cross-section  of  the  spinal  cord  in  the  adult  (Fig.  55).  The 
mantle  layer  with  its  many  nuclei  differentiates  into  the  gray  matter  of  the  spinal 
cord,  which  contains  the  nerve-cells  and  their  dendritic  processes.  The  mar- 
ginal layer  develops  into  the  white  substance  as  a  result  of  the  growth  into  it 
of  the  axons  from  neuroblasts  located  within  the  mantle  layer.  These  form 
association  fibers  which  ascend  or  descend  through  the  marginal  layer  and  serve 
to  connect  one  level  of  the  neural  tube  with  another.  It  is  not  until  these 
longitudinally  coursing  axons  develop  myelin  sheaths  that  the  white  substance 
acquires  its  characteristic  coloration. 

The  cavity  of  the  neural  tube  is  relatively  large,  and  at  the  point  marked 
"neural  cavity"  in  Fig.  22  a  groove  is  visible.  This  is  the  sulcus  limitans.  It 
separates  the  dorsal  or  alar  plate  from  the  ventral  or  basal  plate.  The  mantle 
layer  of  the  alar  plate  develops  into  the  dorsal  gray  column  which,  like  the  other 
parts  developed  from  this  plate,  is  afferent  in  function.  The  afferent  fibers, 
growing  into  the  spinal  cord  from  the  spinal  ganglia,  either  terminate  in  this 
dorsal  column  or  ascend  in  the  posterior  part  of  the  marginal  zone  to  nuclei 
derived  from  the  alar  plate  in  the  myelencephalon.  Most  of  the  association 
fibers  which  run  in  the  marginal  layer  have  grown  out  from  neuroblasts  located 
in  the  dorsal  column.  The  mantle  layer  of  the  basal  plate  gives  rise  to  the 
ventral  gray  column.  From  the  neuroblasts  in  this  region  grow  out  the  motor 
fibers  of  the  ventral  roots  and  spinal  nerves. 

From  what  has  been  said  it  will  be  clear  that  the  entire  nervous  system  is 
ectodermal  in  origin.  The  nervous  element  proper  or  neurons  are  derived  from 
the  neuroblasts;  the  supporting  tissue  of  the  brain  and  spinal  cord,  the  neuroglia, 
is  derived  from  spongioblasts;  while  the  neurilemma  of  the  peripheral  nerves  is 
the  product  of  sheath  cells  which  have  migrated  out  from  the  spinal  ganglia 
and  possibly  also  from  the  neural  tube. 


CHAPTER  IV 

NEURONS  AND  NEURON-CHAINS 

Tin  oervous  system  is  composed  of  highly  irritable  cellular  units,  or  neurons, 
linked  together  to  form  conduction  pathways.  In  the  preceding  chapter  we 
have  seen  that  each  neuron  is  the  product  of  a  single  embryonic  cell  or  neuro- 
blast, and  that,  therefore,  the  nerve-cell  with  all  its  processes  constitute-  a  gen- 
etic unit.  In  the  present  chapter,  as  we  examine  the  form  and  internal  struc- 
ture of  the  neurons  and  their  relation  to  each  other,  we  shall  learn  that  they  are 
also  the  structural  and  functional  units  of  the  nervous  system. 

Form. — There  is  the  widest  possible  variation  in  the  shape  of  nerve-cell-, 
but  all  present  -  »me  features  in  common.  About  the  nucleus  there  is  an  accumu- 
lation of  cytoplasm  which  together  with  the  nucleus  forms  what  is  often  called 
the  cell  body.  A  convenient  term  by  which  to  designate  the  circumnuclear 
cytoplasmic  mass  is  perikaryon.  From  the  perikaryon  cytoplasmic  processes 
are  given  off.  some  of  which  may  be  of  great  length.  The  external  form  of 
the  neuron  depends  on  the  shape  of  the  perikaryon  and  on  the  number,  shape, 
and  ramification  of  these  processes.  Since  the  variety  of  forms  is  almost  with- 
out limit,  we  will  content  ourselves  with  studying  a  few  typical  examples. 

The  pyramidal  cells  of  the  cerebral  cortex  are  good  examples  (Fig.  23).  The 
perikarvon  is  triangular  in  form.  One  angle,  that  directed  toward  the  surface 
of  the  cortex,  is  prolonged  in  the  form  of  a  long  thick  branching  process,  the 
apical  dendrite.  From  the  sides  and  other  angles  of  the  perikaryon  arise  shorter 
branching  dendrites,  while  from  the  base  or  from  one  of  the  basal  dendrite- 
uri-es  a  long  slender  process,  the  axon.  The  characteristic  features  of  the  den- 
drites are  as  follows:  they  branch  repeatedly,  rapidly  decrea-e  in  size,  and 
terminate  not  far  from  the  cell  body.  Their  contour  is  irregular  and  they  are 
studded  with  short  side  branches,  or  gemmules.  which  give  them  a  spiny  appear- 
ance. Each  neuron  usually  possesses  several  dendrites,  but  in  some  types  oi 
nerve-cells  they  are  absent  altogether.  The  axon,  on  the  other  hand,  is  char- 
acterized by  its  uniform  smooth  contour,  relatively  small  diameter,  and  in  most 
instances  by  its  great  length  and  relative  freedom  from  side  branches.  It  may 
give  off  fine  side  branches,  or  collaterals,  near  its  origin;  and  these  arise  at  right 

43 


44 


1111.    M.k\  I  'I  -    SYS1  I.M 


Fig.  23. — A  pyramidal  cell  from  the  cere- 
bral cortex  of  a  mouse:  a,  Dendrites  from  the 
base  of  the  cell;  b,  white  substance  of  the 
hemisphere  into  which  the  axon,  e,  can  be 
traced;  c,  collaterals  from  the  first  part  of  the 
axon;  /,  apical  dendrite;  p,  its  terminal 
branches  near  the  surface  of  the  cortex. 
Golgi  method.     (Cajal.) 


cerebrospinal  ganglia  (Fig.  40). 


angles  to  the  parent  stem.  The  axon 
terminates  in  a  multitude  of  fine  branches 
usually  at  a  considerable  distance  and 
sometimes  as  much  as  a  meter  from  its 
origin.  The  origin  of  the  axon  from  the 
perikaryon  is  marked  by  an  expansion 
known  as  the  cone  of  origin  or  im- 
plantation cone.  This  cone,  like  the 
axon,  differs  somewhat  in  structure  from 
the  perikaryon.  Such  long  axons  as 
have  just  been  described  are  character- 
istic of  the  cells  of  Golgi's  Type  I. 

That  not  all  axons  are  long  and 
relatively  unbranched  is  seen  from  Fig. 
24.  which  illustrates  a  cell  of  Gold's 
Type  II.  The  axons  of  these  cells  are 
short,  branch  repeatedly,  and  end  in  the 
neighborhood  of  the  cell  body. 

Another  good  example  is  furnished 
by  the  primary  motor  neurons.  Figure 
25  illustrates  such  a  cell  from  the  anterior 
gray  column  of  the  spinal  cord.  This 
is  a  large  nerve-cell  with  many  rather 
long  branching  dendrites  and  an  axon. 
which  forms  the  axis-cylinder  of  a  motor 
nerve-fiber  and  terminates  by  forming 
a  motor  ending  in  a  muscle.  As  illus- 
trated in  this  figure,  long  axons  tend  to 
acquire  myelin  sheaths,  and  those 
which  run  in  the  cerebrospinal  nerves 
are  also  covered  by  a  nucleated  mem- 
branous sheath — the  neurilemma. 

Xerve-cells    with    many    proce- 
such  as  have  just  been  described,  are 
called  multipolar.     Examples  of  unipo- 
lar and  bipolar  cells  are  furnished  by  the 
These  cells,  which  will  be  described  in  more 


\i  i  RONS    AND    Ml  RON  <  II  \l\s 


45 


detail  in  another  chapter,  arc  devoid  of  dendrites.    The  axon  of  such  a  unipolar 
cell  divides  dichotomously  into  a  central  and  a  peripheral  branch,  cadi  po 
ing  the  characteristics  of  an  axon. 

li  is  not  uncommon  to  regard  the  peripheral  branch  of  a  sensory  neuron  as  a  dendrite, 
because  like  the  dendrites  ii  conducts  nerve  impulses  toward  the  cell  body.  But,  since  it 
possesses  all  the  morphologic  characteristics  of  an  axon,  and  since  any  axon  is  able  to  con- 
duct nerve  impulses  throughout  its  length  in  either  direction,  and  since  these  peripheral 

branches  Of  the  sensory   neurons  actually  convey  impulses  distally  in   the  phenomenon  ol 


Fig.  24. — Neurons  with  short  axons  (Type  II  of  Golgi)  from  the  cerebral  cortex  of  a  child:   a, 

Axon.     Golgi  method.     (Cajal.) 

antidromic  conduction  (Bayliss,  General  Physiology,  p.  474),  it  seems  best  to  consider  both 
central  and  peripheral  branches  as  divisions  of  a  common  axonic  stem.  (See  Barker,  The 
Nervous  System,  p.  361.) 


From  what  has  been  said  it  will  be  apparent  that  a  neuron  usually  possesses 
several  dendrites  and  a  single  axon,  but  some  have  only  one  process,  which  is 
then  an  axon.     It  may  be  added  that  some  neurons  have  more  than  one  axon. 

Nerve-fibers  are  axons  naked  or  insheathed.  Two  myelinated  peripheral 
nerve-fibers  are  shown  in  Fig.  26.     The  axon  or  axis-cylinder  is  composed  of 


46 


THE    NERVOUS    SYSTEM 


delicate  neurofibrils  embedded  in  a  semifluid  neuroplasm.     It  is  surrounded  by 
a  relatively  thick  myelin  sheath  and  a  nucleated  membranous  neurilemma  sheath. 


Fig.  25. — Primary  motor  neuron  (diagram- 
matic): ah,  Implantation  cone  of  axon;  ax, 
axon;  c,  cytoplasm;  d,  dendrites;  w,  myelin 
sheath;  m',  striated  muscle;  n,  nucleus;  »', 
nucleolus;  nR,  node  of  Ranvier;  sf,  collateral; 
si,  neurilemma;  tel,  motor  end-plate.  (Barker, 
Bailey.) 


Fig.  26. — Portions  of  two  nerve-fibers 
stained  with  osmic  acid  (from  a  young  rabbit). 
Diagrammatic.  425  diameters:  RR,  Nodes  of 
Ranvier,  with  axis-cylinder  passing  through;  a, 
neurilemma;  c,  opposite  the  middle  of  the  seg- 
ment, indicates  the  nucleus  and  protoplasm  ly- 
ing between  the  neurilemma  and  the  medullary 
sheath.  In  A  the  nodes  are  wider,  and  the  in- 
tersegmental substance  more  apparent  than  in 
B.     (Schafer,  in  Quain's  Anatomy.) 


The  myelin  sheath  consists  of  a  fatty  substance,  myelin,  supported  by  a  retic- 
ulum of  neurokeratin.  The  latter,  not  seen  in  the  living  fiber,  may  be  a  coag- 
ulation product  produced  during  fixation.     The  highly  refractive  myelin  gives 


\l  I  RONS    AMi    \ll  RON  '  BAINS 


47 


to  the  myelinated  fibers  a  whitish  color.  This  sheath  is  interrupted  al  regular 
intervals  by  constrictions  in  the  nerve  fiber  known  as  the  nodes  of  Ranvier. 
The  constrictions  are  produced  by  a  dipping  in  of  the  neurilemma  sheath  toward 
the  axon,  which  runs  without  interruption  through  the  node.  The  part  of  a 
fiber  between  each  node  is  an  internoda]  segment,  and  each  such  segmenl  pos 
sesses  a  nucleus  which  is  surrounded  by  a  small  amount  of  cytoplasm  and  lies 
jusl  beneath  the  neurilemma.  The  latter  is  a  thin  membranous  outer  covering 
tor  the  fiber.  Each  segment  of  the  neurilemma  sheath,  together  with  the  cell 
which  lies  beneath,  is  the  product  of  a  single  sheath  cell  of  ectodermal  origin. 
Fibers  such  as  have  just  been  described  are  found  in  the  cerebrospinal  nerves, 
and  give  these  their  white  glistening  appearance. 

The  myelinated  fibers  of  the  brain  and  spinal  cord  are  of  somewhat  different 
structure.  There  is  no  evidence  of  segmentation  in  the  myelin  sheath  and 
neither  the  neurilemma  nor  its  cells  are  present.  This  fact  is  of  much  im- 
portance in  the  phenomena  of  regeneration,  as  will  be  explained  later.  These 
are  the  fibers  which  give  the  characteristic  color  to  the  white  matter  of  the 
brain  and  spinal  cord. 

Unmyelinated  fibers  are  of  two  kinds,  namely,  Remak's  fibers  and  naked 
axons.  The  former  possess  nuclei  which  may  be  regarded  as  belonging  to  a 
thin  neurilemma.  They  are  found  in  great  numbers  in  the  sympathetic  nervous 
system,  and  many  of  the  fine  afferent  fibers  of  the  cerebrospinal  nerves  also 
belong  to  this  class  (Ranson,  1911).  Naked  axons  are  especially  numerous  in 
the  gray  matter  of  the  brain  and  spinal  cord,  and  it  may  be  added  that  every 
axon  at  its  beginning  from  the  nerve-cell,  as  well  as  at  its  terminal  arborization, 
is  devoid  of  covering. 

By  way  of  summary  we  may  enumerate  four  kinds  of  nerve-fibers:  (1)  myelin- 
ated fibers  with  a  neurilemma,  found  in  the  peripheral  nervous  system,  especially 
in  the  cerebrospinal  nerves;  (2)  myelinated  fibers  without  a  neurilemma,  found 
in  the  central  nervous  system;  (3)  unmyelinated  fibers  with  nuclei  (Remak's 
fibers),  especially  numerous  in  the  sympathetic  system,  and  (4)  naked  axons, 
abundant  in  the  gray  matter  of  the  brain  and  spinal  cord. 

Neuroglia  cells  and  fibers  will  be  considered  in  connection  with  the  structure 
of  the  spinal  cord. 

Structure  of  Neurons. — Like  other  cells,  a  neuron  consists  of  a  nucleus  sur- 
rounded by  cytoplasm,  and  these  possess  the  fundamental  characteristics  which 
belong  to  nucleus  and  cytoplasm  everywhere,  but  each  presents  certain  features 
more  or  less  characteristic  of  the  nerve-cell.     The  nucleus  is  large  and  spheric; 


48  THE    NERVOUS    SYSTEM 

and,  because  it  contains  little  chromatin,  it  stains  lightly  with  the  basic  dyes 
(Fig.  27,  .1).  It  contains  a  large  spheric  nucleolus.  The  cytoplasm,  enclosed 
in  a  cell  membrane,  is  characterized  by  the  presence  of  basophil  granules  and 
a  fibrillar  reticulum.  The  granules,  which  apparently  are  a  product  of  the 
nucleus,  are  composed  of  nucleoprotein.  They  are  grouped  in  dense  clumps, 
known  as  Nissl  bodies  or  tigroid  masses,  and  stain  deeply  with  methylene-blue. 
The  size,  shape,  and  arrangement  of  the  Xissl  bodies  differ  with  the  type  of 
nerve-cell  studied.  They  are  much  larger  in  motor  than  in  sensory  neurons 
(Malone,  1913).  While  they  are  found  in  the  larger  dendrites,  the  axon  and 
its  cone  of  origin  are  free  from  them.     They  are  intimately  concerned  in  the 


A  xon 


Fig.  27. — Nerve-cells  stained  with  toluidin  blue:  A,  From  anterior  horn  of  spinal  cord  of  the 
monkey,  shows  Xissl  bodies  in  cytoplasm;  B,  from  the  facial  nucleus  of  a  dog,  shows  a  partial 
disappearance  of  the  Nissl  bodies  (chromatolysis)  resulting  from  section  of  the  facial  nerve. 
(Schafer.) 

metabolic  activity  of  the  cell,  increasing  during  rest  and  decreasing  as  a  result 
of  fatigue.  They  also  undergo  solution  as  a  result  of  injury  to  the  axon  even 
at  a  great  distance  from  the  cell,  the  so-called  axon-reaction  or  chromatolysis 
(Fig.  27,  B). 

The  neurofibrils  were  first  brought  forcefully  to  the  attention  of  neurologists 
by  Bethe  (1903).  These  are  delicate  threads  which  run  through  the  cytoplasm 
in  every  direction  and  extend  into  the  axon  and  dendrites  (Fig.  28).  The 
appearance  of  the  fibrillae  differs  according  to  the  technic  employed  in  preparing 
the  tissue  for  microscopic  examination.  While  in  the  preparations  by  Bethe's 
method  the  fibrils  do  not  appear  to  branch  or  anastomose  with  each  other,  those 
seen  in  Cajal  preparations  divide,  and  by  anastomosing  with  each  other  form 


M  I   R(  INS     WI>    M  i   Rl  IN  ■  II  \I\S 


49 


a  true  network.    The  fibrillar  can  be  traced  to  the  terminations  oi  the  dendrites 
and  axons.     They  have  been  looked  upon  by  many  as  the  chief  elements  in 
volved  in  the  conduction  of  the  nerve  impulse; 

Other  elements  such  as  pigment  granules  may  De  present.  Mitochondria 
have  been  described  in  nerve  cells  by  Cowdrj  1 191  1 1  and  Rasmussen  1 1919 

Interrelation  of  Neurons.  In  the 
ccelenterates,  as  we  have  learned,  a  single 
nerve-cell  may  receive  the  stimulus  and 
transmit  it  to  the  underlying  muscle. 
But  in  vertebrates  the  transmission  of  a 
nerve  impulse  to  an  effector  requires 
a  chain  of  at  least  two  neurons,  the  im- 
pulse parsing  from  one  neuron  to  the  next 
along  the  chain.  One  of  the  most  im- 
portant problems  in  neurology,  there- 
fore, is  this:  How  are  the  neurons  re- 
lated to  each  other  so  that  the  impulse 
may  be  propagated  from  one  to  the 
other?  The  place  where  two  such  units 
come  into  such  functional  relation  is 
known  as  a  synapse.  In  a  synapse  the 
axon  of  one  neuron  terminates  on  the 
cell  body  or  dendrites  of  another.  Func- 
tional connections  are  never  established 
between  the  dendrites  of  one  neuron 
and  the  cell  body  or  dendrites  of  an- 
other. In  Fig.  29  the  axon  of  a  basket 
cell  of  the  cerebellum  is  seen  giving  off 
collaterals  which  terminate  about  and 
form  synapses  with  the  Purkinje  cells. 
Another  type  of  synapse  is  illustrated  in  denhain.) 
Fig.  70. 

The  processes  of  one  nerve-cell  are  not  directly  fused  with  those  of  others, 
but,  on  the  contrary,  each  neuron  appears  to  be  a  distinct  anatomic  unit.  At 
least  the  most  detailed  study  of  Golgi  and  Cajal  preparations,  in  which  the 
finest  ramifications  of  dendrites  and  axons  are  stained,  has  failed  to  demon- 
strate a  structural  continuity  between  neurons.     In  especially  favorable  material 


AW 


Fig.  28. — Neurofibrils  in  a  cell  from  the 
anterior  gray  column  of  the  human  spinal 
cord:  <;.v,  Axon;  lii,  interfibrillar  spaces 
nucleus;  .r,  neurofibrils  passing  from  one 
dendrite  to  another;  y,  neurofibrils  passing 
through   the   body  of  the  cell.     (Bet lie,  llei- 


5° 


iiiK  xkrvoi  s  systkm 


Bartelmez  (1915)  has  shown  that  an  axon  and  dendrite,  entering  into  the  forma- 
tion of  a  synapse,  are  each  surrounded  by  a  distinct  plasma  membrane  and 
that  there  is  no  direct  protoplasmic  continuity.  It  has  been  maintained  by 
Bethe  and  others  that  at  such  points  of  contact  the  neurofibrils  pass  without 
interruption  from  one  neuron  to  another,  but  this  has  been  denied  by  Cajal. 
The  relation  between  two  neurons  at  a  synapse  appears  to  be  one  of  contact, 
but  not  of  continuity  of  substance. 

Nerve  impulses  pass  across  the  synapse  in  one  direction  only,  i.  e.,  from  the 
axon  to  the  adjacent  cell  body  or  dendrite.  As  a  corollary  of  this  it  is  obvious 
that  impulses  must  travel  within  the  neuron  from  dendrites  to  perikaryon  and 
then  out  along  the  axon,  as  indicated  by  the  arrow  in  Fig.  30.     This  is  known 


Fig.  29. — Basket  cell  from  the  cerebellar  cortex  of  the  white  rat.     The  Purkinje  cells  are  indicated 

in  stipple.     Golgi  method.     (Cajal.) 

as  the  law  of  dynamic  polarity.  The  polarity  is,  however,  not  dependent  upon 
anything  within  the  neuron  itself,  but  upon  something  in  the  nature  of  the 
synaptic  interval  which  permits  the  impulses  to  travel  across  it  in  one  direc- 
tion only.  There  are  many  lines  of  evidence  which  indicate  that  when  once 
activated  a  nerve-fiber  conducts  equally  well  in  either  direction.  When  a  motor 
fiber  bifurcates,  sending  a  branch  to  each  of  two  separate  muscles,  stimulation 
of  one  branch  will  cause  an  impulse  to  ascend  to  the  point  of  bifurcation,  and 
then  descend  along  the  other  branch  to  its  motor  ending  (Fig.  30).  This  can 
often  be  demonstrated  in  regenerated  nerves  (Feiss,  1912).  The  phenomena 
of  antidromic  conduction  and  the  axon  reflex  (Bayliss,  1915)  are  also  explained 
by  the  assumption  that  impulses  are  able  to  travel  along  a  nerve-fiber  in  either 
direction. 


M  I  RONS    AND    \i  I  RON  <  II  \i\S  51 

The  Neuron  as  a  Trophic  Unit.  All  parts  "I"  a  cell  are  interdependent,  and  a 
continuous  interaction  between  the  nucleus  and  cytoplasm  is  a  necessan  con 
dition  for  life.  An\  part  which  is  detached  from  the  portion  containing  the 
nucleus  will  disintegrate.  In  this  respect  the  nerve  cell  is  no  exception.  When 
an  axon  is  divided,  that  part  which  is  separated  from  its  cell  of  origin  and 
therefore  from  its  nucleus  dies,  while  the  part  -till  connected  with  the  cell 
usually  survives.  The  degeneration  of  the  distal  fragment  of  the  axon  extends 
to  its  finest  ramifications,  but  does  not  pass  the  synapse  nor  involve  the  next 
neuron. 

It  must  not  be  supposed,  however,  that  the  part  of  the  neuron  containing  the 
nucleus  remains  intact,  for  as  a  result  of  the  division  of  an  axon  important 


Motor  neuron 


Sensory  neuron 


Fig.  30. — Diagram  of  a  reflex  arc  to  illustrate  the  law  of  dynamic  polarity.     The  arrows  indicate 

the  direction  of  conduction. 

changes  occur  in  the  cell  body.  The  Nissl  bodies  undergo  solution,  the  cell 
becomes  swollen,  and  the  nucleus  eccentric.  This  phenomenon  is  known  as 
chromatolysis,  or  the  axon  reaction,  and  is  illustrated  in  Fig.  27,  B.  If  the 
changes  have  been  very  profound  the  entire  neuron  may  completely  disin- 
tegrate; but,  as  a  rule,  it  is  restored  to  normal  again  by  reparative  processes. 
The  nucleus  becomes  more  central,  the  Nissl  bodies  reform  and  usually  become 
more  abundant  than  before,  while  from  the  cut  end  of  the  axon  new  sprouts 
grow  out  to  replace  the  part  of  the  axon  which  has  degenerated.  From  what 
has  been  said  it  will  be  apparent  that  the  nucleus  presides  over  the  nutrition  of 
the  entire  neuron,  that  the  latter  responds  as  a  whole  to  an  injury  of  even  a 
distant  part  of  its  axon,  that  the  changes  produced  by  such  a  lesion  are  limited 
to  the  neuron  directly  involved,  and  that  nerve-fibers  are  unable  to  maintain 


52  THE    NERVOUS    SYSTEM 

a  separate  existence  or  to  regenerate  when  their  continuity  with  the  cell  body 
has  been  lost.  This  is  what  is  meant  by  the  statement  that  the  neuron  is  the 
trophic  unit  of  the  nervous  system. 

Degeneration  and  Regeneration  of  Nerve-fibers.— A-  has  already  been  stated, 
that  portion  of  a  divided  fiber  which  has  been  separated  from  its  cell  of  origin 
degenerates.  The  axon  breaks  up  into  granular  fragments,  the  myelin  under- 
goes chemical  change  and  forms  irregular  fatty  globules.  Later  the  degenerated 
axon,  and  myelin  are  entirely  absorbed.  The  neurilemma  cells  of  a  degener- 
ated peripheral  nerve-fiber  increase  in  number,  their  cytoplasm  increases  in 
quantity,  and  they  become  united  end  to  end  to  form  nucleated  protoplasmic 
bands  or  band-fibers.  These  changes  in  the  nerve-fiber  are  known  as  Wallerian 
degeneration. 

In  regeneration  new  axons  grow  out  from  the  old  ones  in  the  central  unde- 
generated  portion  of  the  nerve.  These  grow  into  the  distal  degenerated  stump 
and  find  their  way  along  the  nucleated  protoplasmic  bands,  mentioned  above, 
to  the  terminals  of  the  degenerated  nerve.  These  band-fibers  serve  as  conduits 
for  the  growing  axons  and  from  them  the  new  neurilemma  sheaths  are  differ- 
entiated. Thus,  while  the  neurilemma  cells  and  the  band-fibers  derived  from 
them  appear  to  be  incapable  of  developing  new  nerve-fibers  by  themselves  in 
the  peripheral  stump,  they  play  an  important  part  in  nerve  regeneration  in 
co-operation  with  the  new  axons  from  the  central  stump  (Cajal,  1908;  Ranson, 
1912).  It  is  important  to  note  that  the  nerve-libers  of  the  brain  and  spinal 
cord,  which,  as  has  been  stated  before,  are  devoid  of  neurilemma  sheaths,  are 
incapable  of  regeneration. 

The  neuron  concept,  which  is  based  on  such  facts  as  have  been  presented 
in  the  preceding  paragraphs,  was  first  clearly  formulated  by  Waldeyer  in  1891, 
who  was  also  the  first  to  use  the  name  neuron  for  the  elements  under  considera- 
tion.    The  neuron  doctrine  may  be  summarized  as  follows: 

1.  The  neuron  is  the  genetic  unit  of  the  nervous  system — each  being  derived 
from  a  single  embryonic  cell,  the  neuroblast. 

2.  The  neuron  is  the  structural  unit  of  the  nervous  system,  a  nerve-cell  with 
all  its  processes.  These  cellular  units  remain  anatomically  separate,  i.  c.  while 
they  come  into  contact  with  each  other  at  the  synapses  there  is  no  continuitv 
of  their  substance. 

3.  The  neurons  are  the  functional  units  of  the  nervous  system.  Thev  are 
conduction  units  and  the  conduction  pathways  are  formed  of  chains  of  such 
units. 


NEURONS    \\i>    mi  RON  <  II  \l\s 


53 


4.  The  neuron  is  also  a  trophic  unit,  as  is  -ecu  (a)  in  the  degeneration  of  a 
portion  of  an  axon  severed  from  its  cell  of  origin,  (b)  in  the  phenomenon  of 
chromatolysis  or  axon  reaction,  and  (c)  in  the  regeneration  of  the  degenerated 
portion  ol  the  axon  by  an  outgrowth  from  that  part  of  the  axon  still  In  con 
tact  with  its  cell  of  origin. 

5.  Neurons  are  the  only  elements  concerned  in  the  conduction  of  nerve 
impulses.  The  nervous  system  is  composed  of  untold  numbers  of  such  unit- 
linked  together  in  conduction  systems. 

While  a  majority  of  neurologists  now  accept  the  neuron  doctrine  as  pre- 
sented here,  there  are  dissenters  (Marui,  1918).     In  his  very  interesting  book, 


Fig.  31. — Diagrammatic  section  through  the  spinal  cord  and  a  spinal  nerve  to  illustrate  a 
simple  reflex  arc:  a,  b,  c,  and  d,  Branches  of  sensory  fibers  of  the  dorsal  roots;  e,  association  neuron; 
/,  commissural  neuron. 


"Allgemeine  Anatomie  und  Physiologie  des  Nervensystems,"  Bethe  has  vigor- 
ously controverted  every  one  of  the  five  cardinal  points  just  presented. 

We  will  next  examine  some  of  the  simpler  chains  of  neurons  to  see  how  they 
enter  into  the  formation  of  the  conduction  pathways. 

Neuron-chains.— The  simplest  functional  combination  of  neurons  is  seen  in 
the  reflex  arc,  and  this  again  in  its  simplest  form  is  illustrated  in  Fig.  31.  Such 
an  arc  may  consist  of  but  two  neurons,  one  of  which  is  afferent  and  conducts 
toward  the  spinal  cord;  the  other  is  efferent  and  conducts  the  impulses  to  the 
organ  of  response.  The  arc  consists  of  the  following  parts:  (1)  the  receptor, 
the  ramification  of  the  sensory  fiber  in  the  skin  or  other  sensory  end  organ; 
(2)  the  first  conductor,  which  includes  both  branches  of  the  axon  of  the  spinal 
ganglion  cell;  (3)  a  center  including  the  synapse;  (4)  the  second  conductor,  which 


54 


Mil     NERVOUS    SYSTEM 


includes  the  entire  motor  neuron,  with  its  cell  body  in  the  anterior  gray  column 
and  its  motor  ending  on  the  muscle,  and  (5)  the  effector  or  organ  of  response, 
which  in  this  case  is  a  muscle-fiber.  A  wave  of  activation,  known  as  the  nerve 
impulse,  is  developed  in  the  sensitive  receptor,  travels  over  this  arc,  and  on 
reaching  the  muscle  causes  it  to  contract.  So  simple  a  reflex  is  rare,  but  prob- 
ably the  knee-jerk  is  an  example  (Jolly,  1911).  A  more  common  form  of  reflex 
arc  involves  a  third,  and  purely  central  neuron,  as  illustrated  on  the  right  side 
of  Fig.  31.  Such  central  elements  may  be  spoken  of  as  association  and  com- 
missural neurons.     Many  of  them  serve  to  connect  distant  parts  of  the  central 


Fig.  32. — Diagram  representing  some  of  the  conduction  paths  through  the  mammalian  central 
nervous  system.  An  elaborate  system  of  central  or  association  neurons  furnishes  a  number  of 
alternative  paths  between  the  primary  sensory  and  motor  neurons.     (Redrawn  from  Bayliss.) 

nervous  system  with  each  other  (Fig.  68).  It  is  to  the  multiplication  of  these 
central  neurons  that  we  owe  the  complicated  pathways  within  the  mammalian 
brain  and  spinal  cord. 

Pathways  Through  Higher  Centers. — A  good  idea  of  how  the  neurons  of  some 
of  the  centers  in  the  brain  are  related  to  the  primary  motor  and  sensory  spinal 
neurons  is  given  by  Fig.  32.  It  will  be  seen  that  many  paths  are  open  to  an 
impulse  entering  the  spinal  cord  by  way  of  a  dorsal  root  fiber:  (1)  It  may  pass 
by  way  of  a  collateral  to  a  primary  motor  neuron  in  a  two-neuron  reflex  arc. 
It  may  travel  over  an  association  neuron,  belonging  (2)  to  the  same  level  of  the 


M  i  !■'  »NS    AND    \i  i  RON  I  II  \I\s 


55 


spinal  o>nl.  or  (3)  to  other  levels,  in  reflex  arcs  of  three  or  more  neurons  each; 
or  I  it  may  ascend  to  the  brain  along  an  ascending  branch  of  a  dorsal  rool 
fiber.  Here  it  may  travel  over  one  or  more  of  a  number  of  path-,  each 
sisting  of  several  neurons,  and  be  anally  returned  to  the  spinal  cord  and  make- 
it-  exit  by  way  of  a  primary  motor  neuron.  The  figure  illustrates  but  a  few  of 
the  possible  paths,  many  of  which  we  shall  have  occasion  to  consider  in  the 
subsequenl  chapters. 

For  an  incoming  impulse  a  variety  of  paths  arc  open,  one  or  more  of  which 
may  be  taken  according  to  the  momentary  resistance  of  each.  There  is  reason 
to  believe  that  the  resistance  to  conduction  across  a  synapse  may  vary  from 
moment  to  moment,  according  to  the  physiologic  state  of  the  neuron-  involved. 
It  is  therefore  not  necessary  that  every  impulse  entering  by  a  given  fiber  shall 
travel  the  same  path  within  the  central  nervous  system  nor  produce  the  same 
result.  The  pathways  themselves  are,  however,  more  or  less  fixed,  and  depend 
upon  the  structural  relations  established  among  the  neurons.  Many  of  these 
synaptic  connections  are  formed  before  birth,  follow  an  hereditary  pattern, 
and  are  approximately  the  same  for  each  individual  of  the  species.  In  the  child 
these  are  illustrated  by  the  nervous  mechanisms  involved  in  breathing  and 
swallowing,  which  are  perfect  at  birth.  The  newly  hatched  chick  is  able  to  run 
about  and  pick  up  food,  acts  which  are  dependent  on  nervous  connections  al- 
ready established  according  to  hereditary  pattern.  In  man  and  to  a  less  extent 
in  other  mammals  the  nervous  system  continues  to  develop  long  after  birth. 
This  postnatal  development  is  influenced  by  the  experience  of  the  individual 
and  is  more  or  less  individual  in  pattern.  It  is  probable  "that  in  certain  parts 
of  the  nervous  mechanism  new  connections  can  always  be  established  through 
education"  (Edinger,  1911). 

The  neurons  wrhich  make  up  the  nervous  system  of  an  adult  man  are  there- 
fore arranged  in  a  system  the  larger  outlines  of  which  follow  an  hereditary  pat- 
tern, but  many  of  the  details  of  which  have  been  shaped  by  the  experiences  of 
the  individual. 


CHAPTER  V 

THE  SPINAL  NERVES 

We  have  had  a  glance  at  the  earliest  beginnings  of  a  nervous  system  in  the 
animal  series  and  learned  something  of  its  biologic  significance.  We  have 
traced  briefly  its  development  in  the  mammalian  embryo,  and  become  familiar 
with  its  chief  subdivisions.  We  have  studied  the  microscopic  units  of  which  it 
is  composed,  learning  something  of  their  development,  structure,  and  function. 
With  this  information  we  are  prepared  to  take  up  a  more  detailed  study  of  the 
various  subdivisions  of  the  system. 

Subdivisions  of  the  Nervous  System. — The  most  convenient  and  logical 
classification  of  the  parts  of  the  nervous  system  is  that  which  emphasizes  the 
distinction  between  the  central  organs  and  those  peripheral  portions  which  are 
concerned  chiefly  in  conducting  impulses  to  and  from  the  central  organs,  as 
follows : 

The  central  nervous  system : 
Brain.    • 
Spinal  cord. 
The  peripheral  nervous  system : 
Cerebrospinal  nerves: 
Cranial  nerves. 
Spinal  nerves. 
The  sympathetic  nervous  system. 
The  anatomic  relationships  of  these  subdivisions  in  man  are  illustrated  in 
Figs.  33  and  34.     The  brain  lies  within  and  nearly  fills  the  cranial  cavity.     It  is 
continuous  through  the  foramen  magnum  with  the  spinal  cord,  which  occupies 
but  does  not  fill  the  vertebral  canal.     From  the  brain  arises  a  series  of  nerves 
usually  enumerated  as  twelve  pairs  and  known  as  cranial  01  cerebral  nerves; 
while  thirty-one  pairs  of  segmentally  arranged  spinal  nerves  take  origin  from  the 
spinal  cord. 

Branches  of  the  cerebrospinal  nerves  reach  most  parts  of  the  body.     They 

are  composed  of  afferent  fibers,  which  receive  and  carry  to  the  central  nervous 

system  sensory  impulses  produced  by  external  or  internal  stimuli,  and  of  efferent 

fibers,  which  convey  outgoing  impulses  to  the  organs  of  response.     It  is  through 

56 


i in:  spina i.  NERVES 


57 


the  centra]  nervous  system  that  the  incoming  impul  e  find  their  way  into  the 
proper  outgoing  paths.  To  bring  about  this  shunting  oi  incoming  impulses 
into  the  appropriate  efferent  paths  requires  the  presence  of  untold  numbers 

( 'Mary  ganglion        Maxillary  nerve 
Sphenopalatine  ganglion 

Superior  cerokal  ganglion  of  sympathetic 

•P 


Cervical 
plexus 


Brachial 
plexus 


Greater  Jffi 
splanchnic  nerve  ~~^K 

Lesser 

splanchnic  nerve 


Lumbar 
plexus 


Sacral 
pie  cus 


Pharyngeal  plexus 

Middle  cervical  ganglion  of  sympathetic 
Inferior  cervical  gang,  of  sympathetic 
Recurrent  nervt 
Bronchial  plexus 

Cardiac  plexus 


Esophageal  plexus 
^Coronary  plexu  i 


Left  vagus  nerve 

Gastric  plexus 
Celiac  plexus 

Superior  mesenteric  plexus 


—j- Aortic  plexus 
^—Inferior  mesenteric  plexus 

Hypogastric  plexus 

Pelvic  plexus 

Bladder 
Vesical  plexus 


Fig.  33.  Fig.  34. 

Fig.  33.— General  view  of  the  central  nervous  system,  showing  the  brain  and  spinal  cord  in  situ. 

(Bourgery,  Schwalbe,  van  Gehuchten.) 
Fig.  34.— Diagram  of  the  sympathetic  nervous  system  and  its  connections  with  the  cerebrospinal 

nerves.     (Schwalbe,  Herrick.) 

of  central  or  association  neurons,  and  it  is  of  these  that  the  central  organs- 
brain  and  spinal  cord — are  chiefly  composed. 

Many  authors  employ  a  classification  which  emphasizes  the  distinction  be- 


58  THE   NERVOUS    SYSTEM 

tween  the  cerebrospinal  nervous  system,  composed  of  the  brain  and  spinal  cord 
with  their  associated  nerves,  and  the  sympathetic  nervous  system.  But  this  usage 
has  the  disadvantage  that  it  is  likely  to  engender  an  entirely  false  notion  of  the 
independence  of  the  sympathetic  system. 

The  spinal  nerves  take  origin  from  the  spinal  cord  within  the  vertebral  canal 
and  make  their  exit  from  this  canal  through  the  corresponding  intervertebral 
foramina.  As  component  parts  of  such  a  nerve  there  may  be  recognized  a 
ventral  and  a  dorsal  ramus,  a  ventral  and  a  dorsal  root,  and  associated  with 
the  latter  a  spinal  ganglion.  The  fibers  of  the  ventral  root  have  their  cells  of 
origin  within  the  spinal  cord  and  are  distributed  through  both  ventral  and 
dorsal  rami.  Since  they  conduct  impulses  from  the  spinal  cord  they  are  known 
as  efferent  or  motor  fibers.  The  sensory  or  afferent  fibers  of  the  dorsal  roots 
and  spinal  nerves  arise  from  cells  located  in  the  spinal  ganglia.  These  fibers 
are  also  distributed  through  both  ventral  and  dorsal  rami  (Fig.  37). 

Metamerism. — That  the  spinal  nerves  are  segmentally  arranged,  a  pair  for 
each  metamere,  is  readily  appreciated  in  the  case  of  the  typical  body  segments 
of  the  thoracic  region.  Here  it  is  obvious  that  a  nerve  supplies  the  correspond- 
ing dermatome  and  myotome,  or  in  the  adult  the  skin  and  musculature  of  its 
own  segment.  While  the  thoracic  nerves  retain  this  primitive  arrangement  in 
the  adult,  the  distribution  of  fibers  from  the  other  spinal  nerves  is  complicated 
by  the  development  of  the  limb  buds  and  by  the  shifting  of  myotomes  and 
dermatomes  during  the  development  of  the  embryo. 

Opposite  the  attachment  of  the  limb  buds  the  ventral  rami  of  the  correspond- 
ing nerves  unite  to  form  flattened  plates,  and  from  these  plates  the  brachial  and 
lumbosacral  plexuses  are  developed.  Within  these  plexuses  the  fibers  derived 
from  a  number  of  ventral  rami  are  intermingled  in  what  appears  at  first  to  be 
hopeless  confusion.  Each  nerve  which  extends  from  these  plexuses  into  the 
limbs  carries  with  it  fibers  from  more  than  one  spinal  nerve.  To  determine 
the  exact  distribution  of  the  fibers  from  each  segmental  nerve  has  been  a  very 
difficult  problem,  in  the  elucidation  of  which  the  work  of  clinical  neurologists 
has  been  of  the  first  importance.  A  study  of  the  paralyses  and  areas  of  anes- 
thesia, resulting  from  lesions  involving  one  or  more  nerve  roots  within  the  ver- 
tebral canal,  has  contributed  much  toward  its  solution. 

Sherrington  (1894)  attacked  the  problem  of  the  distribution  of  the  sensory 
fibers  by  experimental  methods  on  cats  and  monkeys.  He  found  that  section  of 
a  single  dorsal  root  did  not  cause  complete  anesthesia  anywhere,  and  attributed 
this  result  to  an  overlapping  of  the  areas  of  distribution  of  adjacent  spinal  nerves. 


Mil     SPIN  \l.    M  l:\  ES 


Next,  selecting  a  particular  dorsal  root  for  study,  he  cut  two  or  three  roots 
both  above  and  below  it.  The  zone  in  which  sensation  -til!  existed  and  which 
was  surrounded  by  an  area  of  anesthesia  represented  the  cutaneous  field  of  thai 
particular  root.  He  found  that  each  "sensory  root  field"  overlapped  tho 
adjacent  roots  Fig.  35).  In  the  thoracic  region  each  such  field  has  the  shape  of 
a  horizontal  band  wrapping  half-way  around  the  body  from  the  middorsal  to 
the  midventral  lines  (Fig.  36). 

Sherrington  also  found  that,  although  in  the  plexuses  associated  with  the 
innervation  of  the  extremities  each  segmental  nerve  contributes  sensory  fibers 
to  two  or  more  peripheral  nerves,  the  cutaneous  distribution  of  these  fibers  is 
not  compiled  of  disjointed  patches,  but  forms  a  continuous  field  running  approxi- 
mately parallel  to  the  long  axis  of  the  limb.  The  general  arrangement  of  these 
sensory  root  fields  in  man  is  indicated  on  the  right  side  of  Fig.  36.     On  the 


/b 


Uh 
thoracic 
s<  tuory 

skiri  field. 


4///////////4 


Sd 
thoracic 


5th 
thoracic. 


Fig.  35. — Diagram  of  the  position  of  the  nipple  in  the  sensory  skin  fields  of  the  fourth,  third, 
and  fifth  thoracic  spinal  roots.  The  overlapping  of  the  cutaneous  areas  is  represented.  (Sher- 
rington.) 

opposite  side  is  indicated  the  distribution  of  the  cutaneous  nerves.  It  will  be 
seen  that  in  the  extremities  there  is  no  correspondence  between  the  areas  sup- 
plied by  these  peripheral  nerves  and  those  supplied  by  the  individual  dorsal 
roots.  It  will  also  be  evident  that  the  fibers  of  a  given  dorsal  root  reach  the 
corresponding  sensory  root  field  by  way  of  more  than  one  cutaneous  nerve. 
A  knowledge  of  the  cutaneous  distribution  of  the  various  nerve  roots  is  of  great 
importance  in  enabling  the  clinician  to  determine  the  level  of  a  lesion  of  the 
spinal  cord  or  nerve  roots  within  the  vertebral  canal. 

In  the  same  way  the  shifting  of  muscles  during  embryonic  development  has 
been  accompanied  by  corresponding  changes  in  the  spacial  distribution  of  the 
motor  fibers.  A  familiar  example  is  furnished  by  the  diaphragm,  the  musculature 
of  which  is  derived  from  the  cervical  myotomes  and  which  in  its  descent  carries 
with  it  the  phrenic  nerve.  This  explains  the  origin  of  the  phrenic  from  the 
third,  fourth,  and  fifth  cervical  nerves. 


6o 


THE    NERVOUS    SYSTEM 


If,  as  seems  probable,  the  musculature  of  the  extremities  has  not  developed 
along  mctameric  lines,  there  can  be  no  true  metamerism  of  the  motor  nerves  to 
the  limbs  (Streeter,  1912).  Yet  the  fibers  from  each  ventral  root  are  distributed 
in  a  very  orderly  manner.  As  is  indicated  in  the  table  on  page  77,  almost  every 
long  muscle  receives  libers  from  two  or  more  ventral  roots.  It  will  be  apparent 
that  the  muscles  of  the  trunk  are  innervated  from  the  roots  belonging  to  the 


Great  auricular 

Cutaneous  nerve  of  the  neck 

Supraclavicular  nerves 

Axillary 

Intercostobrachial 

Medial  cutaneous  of  arm 

Posterior  cutaneous  of  arm 

Medial  cutaneous  of  forearm 

M  useuloe  utaneo  us 

Radial 
Median 
Ulnar 

Genitofemoral 

Lateral  cutaneous  of  the  thigh 

Intermediate  cutaneous  rami 

Medial  cutaneous  rami 

Infrapatellar  ramus 

Lateral  sural 

Saphenous 

Superficial  peroneal 

Sural 

Deep  peroneal 

Fig.  36. — Sensory  root  fields  on  the  right,  contrasted  with  the  areas  of  distribution  of  cutaneous 

nerves  on  the  left. 

several  metameres  from  the  myotomes  of  which  these  muscles  developed.  The 
table  shows  in  a  general  way  the  distribution  of  the  fibers  of  the  several  ventral 
roots. 

Functional  Classification  of  Nerve-fibers. — Many  years  ago  Sir  Charles 
Bell  (1811,  1844)  showed  that  the  dorsal  roots  are  sensory  in  function  and  the 
ventral  roots  motor;  and  this  has  been  known  since  then  as  Bell's  law.  He 
recognized  that  sensory  and  motor  fibers  are  distributed  to  the  viscera  as  well  as 


THE    SPIN  \i.    NERVES 


(.1 


to  the  rest  of  the  body.  But  Gaskell  (1886)  was  the  first  to  make  a  detailed 
study  of  the  nerve-fibers  supplying  the  visceral  and  vascular  \\ . 

now  recognize  in  the  spinal  nerves  elements  belonging  to  lour  functionally 
distinct  varieties,  namely,  visceral  afferent,  visceral  efferent,  somatu  afferent,  and 
somatic  efferent  fibers  (Fig.  37). 

Visceral  Components.-  The  fibers  which  innervate  the  visceral  and  vascular 
systems,  including  all  involuntary  muscle  and  glandular  tissue,  possess,  as 
Gaskell  (1886)  pointed  out  many  years  ago,  certain  distingirishing  character- 
istics.    They  arc  all   fine  myelinated   fibers  and  end   in   sympathetic  ganglia 


Somatic  afferent  fiber    )Dorsalroot 


!  Visceral  afferent  fiber! 


$\ — Spinal  ganglion 
Dorsal  ramus 


>  V(  nlral  ramus 


Ramus  communicans 


Sympathetic  ganglion 


A>K.    Visceral  efferent  fiber}  ,.     .     , 
v    s  c        «■      a       i  r)        i  I  enlral  root 
Somatic  efferent  fiber    j 

Postganglionic  fiber 


.Viscus 


Fig.  37. — Diagrammatic  section  through  a  spinal  nerve  and  the  spinal  cord  in  the  thoracic  region 
to  illustrate  the  chief  functional  types  of  peripheral  nerve-fibers. 

from  which  the  impulses  are  relayed  to  involuntary  muscles  and  glands  by  a 
second  set  of  neurons  (Fig.  37).  They  are  usually  designated  as  visceral  efferent 
fibers,  and  they  run  by  way  of  the  white  rami  to  the  sympathetic  ganglia.  It 
is  usually  stated  that  they  are  found  only  in  the  second  thoracic  to  the  second 
lumbar  nerves  inclusive,  but  Langley  (1892)  has  shown  that  in  the  cat.  dog, 
and  rabbit  they  are  present  in  all  the  thoracic  and  the  first  four  lumbar  nerves, 
and  Miiller  (1909)  found  white  rami  associated  with  the  third  and  fourth  lumbar 
nerves  in  man. 

There  are  also  visceral  afferent   fibers  distributed  to  the  thoracic  and   ab- 
dominal viscera  by  way  of  the  white  rami  from  the  thoracic  and  upper  lumbar 


62 


THE    NERVOUS    SYSTEM 


nerves.  These  have  their  cells  of  origin  in  the  spinal  ganglia  and  are  continued 
through  the  dorsal  roots  into  the  spinal  cord  (Fig.  37).  We  shall  have  much 
more  to  say  about  the  visceral  components  of  the  spinal  nerves  in  the  chapter 
on  the  Sympathetic  Nervous  System.  In  the  remaining  pages  of  this  chapter 
we  will  confine  our  attention  to  the  somatic  components,  i.  e.,  to  those  fibers  which 
innervate  the  various  parts  of  the  body  exclusive  of  the  visceral  and  vascular 
systems. 

Somatic  Efferent  Components. — The  skeletal  muscles  are  innervated  by 
myelinated  fibers,  which  are,  for  the  most  part,  of  large  caliber.  The  axis- 
cylinders  of  these  fibers  are  the  axons  of  cells  located  in  the  ventral  part  of  the 
gray  matter  of  the  spinal  cord,  and  they  end  on  the  muscle-fibers  in  special 


Fig.  38. — Nerve-ending  in  muscular  fiber  of  a  lizard  (Lacerta  viridis).  Highly  magnified:  a. 
End-organ  seen  in  profile;  b,  from  the  surface;  s,  s,  sarcolemma;  p,  p,  expansion  of  axis-cylinder. 
Beneath  this  is  granular  protoplasm  containing  a  number  of  large  clear  nuclei  and  constituting 
the  "bed"  or  "sole"  of  the  end-organ.  In  b  the  expansion  of  the  axis-cylinder  appears  as  a  clear 
network,  branching  from  the  divisions  of  the  medullated  fiber.     (Kiihnc  in  Quain's  Anatomy.) 


motor  end- plates.  Such  a  primary  motor  neuron  is  illustrated  in  Fig.  25.  A 
motor  fiber  undergoes  repeated  division  as  it  approaches  its  termination,  but 
each  branch  retains  its  myelin  sheath  until  in  contact  with  the  muscle-fiber. 
At  this  point  this  sheath  terminates  abruptly,  and  the  neurilemma  becomes 
continuous  with  the  sarcolemma  (Fig.  38).  The  terminal  branches  of  the 
axon  are  short,  thick,  and  irregular.  They  lie  immediately  under  the  sarcolemma 
in  a  bed  of  specialized  sarcoplasm  containing  a  number  of  large  clear  nuclei. 
The  wave  of  activation,  which  travels  down  an  axon  as  a  nerve  impulse,  is 
transmitted  through  these  motor  nerve  endings  to  the  muscle  and  initiates  a 
contraction. 

The  Spinal  Ganglia. — Since  the  afferent  fibers  in  the  spinal  nerves  take  their 


nil     SPINAL    NERVES 

origin  from  the  ganglia  on  the  dorsal  root-  we  will  do  well  to  interrupt  for  a 
moment  our  functional  analyses  of  the  spinal  nerves  and  consider  the  struc- 
ture of  these  ganglia. 

The  spinal  ganglia  arc  rather  simple  structures  so  far  as  their  fundan 
plan  is  concerned,  but  in  recent  year-,  chiefly  through  the  studies  of  Cajal 
(1906)  and  Dogiel  (1908),  we  have  learned  to  recognize  in  them  main-  complex 

histologic  details,  the  significance  of  which  is  not  yet  underst 1.     It  has  long 

been  known  that  the  typical  cells  of  the  mammalian  spinal  ganglion  are  uni- 
polar. The  cell  body  is  irregularly  spheric.  The  axon.1  which  is  attached  to 
the  perikaryon  by  an  implantation  cone,  is  coiled  on  itself  in  the  neighborhood 
of  the  cell,  forming  what  is  known  as  a  glomerulus  (Fig.  39,/).  It  then  runs 
into  one  of  the  central  fiber  bundles  of  the  ganglion  and  divides  in  the  form 
of  a  T  or  Y  into  two  branches,  of  which  one  is  directed  toward  the  spinal  cord 
in  the  dorsal  root.  The  other  and  somewhat  larger  branch  is  directed  distally 
in  the  spinal  nerve.  The  cells  vary  greatly  in  size  and  the  diameter  of  the  axon 
varies  with  that  of  the  cell  from  which  it  springs.  An  axon  arising  from  a 
large  cell  usually  forms  a  very  pronounced  glomerulus  and  soon  becomes  en- 
sheathed  with  myelin,  and  this  myelin  sheath  is  continued  along  both  branches 
into  which  it  divides.     The  branching  occurs  at  a  node  of  Ranvier. 

As  was  originally  pointed  out  by  Cajal  (1906)  and  Dogiel  (1908)  and 
recently  emphasized  by  Ranson  (1911)  the  small  cells  of  these  ganglia  give  rise 
to  fine  unmyelinated  fibers.  These  coil  but  little  near  the  cell,  or  the  glomerulus 
ma\-  be  entirely  lacking  (Fig.  39,  a).  They  divide  dichotomously,  just  as  do 
the  myelinated  fibers,  into  finer  central  and  coarser  peripheral  branches.  At 
the  point  of  bifurcation  there  is  a  triangular  expansion  in  place  of  the  constric- 
tion so  characteristic  of  a  dividing  myelinated  fiber.  It  has  been  shown  by 
Hatai  (1902)  and  Warrington  and  Griffith  (1904)  that  the  small  cells  are  con- 
siderably more  numerous  than  the  large  cells,  though  because  of  their  small 
size  they  constitute  a  less  conspicuous  element. 

A  few  cells  retain  the  bipolar  form  characteristic  of  all  the  spinal  ganglion 
cells  at  an  early  stage  of  development  (Figs.  21,  40,  d). 

The  spinal  ganglion  cells  are  each  surrounded  by  a  capsule  or  membranous 
sheath  with  nuclei  on  its  inner  surface  (Fig.  39,  d.f)  which  is  continuous  with 
the  neurilemma  sheath  of  the  associated  nerve-fiber.  The  cells  forming  the 
capsule  are  of  ectodermal  origin,  being  derived  like  the  spinal  ganglion  cells 
themselves  from  the  neural  crest. 

1  See  fine  print,  page  45. 


64 


THE    NERVOUS    SYSTEM 


In  good  methylene-blue  preparations  and  in  sections  stained  by  the  newer 
silver  methods  it  is  possible  to  make  out  many  additional  details  of  structure. 
The  axon  may  split  into  many  branches,  which  subdivide  and  anastomose, 
forming  a  true  network  in  the  neighborhood  of  the  cell  (Fig.  39,  b).  From  this 
network  the  axon  is  again  assembled  and  passes  on  to  a  typical  bifurcation. 
Or  the  axon  may  be  assembled  out  of  a  similar  plexus  which,  however,  is  con- 

I 


Fig.  39. — Neurons  from  the  spinal  ganglion  of  a  dog:  a,  Small  cells  with  unmyelinated  axons; 
b,  c,  d,  e,  and/,  large  cells  with  myelinated  axons;  f,  typical  large  spinal  ganglion  cell  showing 
glomerulus  and  capsule.     The  arrow  points  toward  the  spinal  cord.     Pyridin-silver  method. 

nected  with  the  cell  by  several  roots  (Fig.  39.  c).  Some  of  the  fibers  give  off 
collaterals  terminating  in  spheric  or  pear-shaped  end-bulbs.  Such  an  end  bulb 
may  rest  upon  the  surface  of  its  own  perikaryon  (Fig.  39,  d)  or  elsewhere  in  the 
ganglion.  From  the  body  of  some  cells  short  club-shaped  dendrites  arise,  which, 
however,  terminate  beneath  the  capsule-  which  surround  the  cells. 

Based  on  such  details  as  these  Dogiel  (1908)  has  arranged  the  spinal  ganglion  cells  in 
groups  and  recognizes  eleven  different  types.  Two  of  his  eleven  types  are  of  special  interest. 
The  cells  of  Type  VIII  resemble  the  typical  spinal  ganglion  cell  in  all  respects  except  that 


Mil.    SPINAL    NERVES  6^ 

the  peripheral  branch  of  the  axon  Wreaks  up  within  the  ganglion  into  numerous  myelinated 
fibers,  which  after  Losing  their  sheaths  terminate  in  whal  an-  apparently  sensory  endings. 
The  centra]  branch  runs  apparently  without  division  to  the  spinal  cord.  The  cells  of  I  vpe 
\  I  possess,  in  addition  to  an  axon,  thai  apparent  ly  runs  wit  houl  division  through  the  dorsal 
mot  to  the  spinal  cord,  several  processes  thai  resemble  dendrites,  in  thai  they  divide  re- 
peatedly within  the  ganglion,  bul  resemble  axons  in  their  appearance  and  in  po  i 
myelin  sheaths  (Fig.  40,  b).  These  processes  after  repeated  divisions  become  unmyelinated 
and  end  within  the  ganglion  and  dorsal  root  in  what  appear  to  be  sensory  endings.  Ii  would 
lead  us  too  far  afield  If  we  should  at  tempi  to  summarise  Dogiel's  work.  It  should  be  pointed 
out,  however,  that  he  no  longer  believes  in  the  existence  of  the  cells  whi<  h  he  formerly  de- 
scribed under  the  head  of  spinal  ganglion  cells  of  Type  II  and  which  find  a  cons£i<  uous  plat  e 
in  most  text-books.  He  believes  that  what  he  formerly  described  as  the  branching  fibers 
of  these  cells  are,  in  reality,  the  dendrite -like  branches  of  the  cells  of   I  vpe  XI. 


Dorsa!  root 


Dorsal  ramus 


Ventral  root  ,*' 

Ramus  communicans 

Ventral  ramus 

Fig.  40. — Diagrammatic  longitudinal  section  of  a  spinal  ganglion  and  a  spinal  nerve  (cervical 
or  sacral) :  a,  Small  cells  with  unmyelinated  axons;  b,  cell  of  Dogiel's  type  XI ;  c,  large  cell  possessing 
a  myelinated  axon  and  surrounded  by  a  pericellular  plexus;  d,  bipolar  cell. 


According  to  Dogiel  every  spinal  ganglion  cell  is  surrounded  by  a  network  of 
fine  branching  and  anastomosing  fibers;  and  he  believes  that  these  are  formed 
by  the  ramifications  of  fine  myelinated  and  unmyelinated  fibers  that  have 
entered  the  spinal  ganglion  from  the  sympathetic  nervous  system  through  the 
rami  communicantes.  While  the  origin  of  these  fibers  is  open  to  question,  there 
can  be  no  doubt  that  such  pericellular  networks  exist  on  at  least  a  considerable 
proportion  of  the  cells  and  constitute  an  important  element  in  the  structure  of 
the  ganglion  (Fig.  40,  c). 

The  fiber  bundles  of  the  ganglia  are  composed  of  both  myelinated  and  un- 

5 


66  THE    NERVOUS    SYSTEM 

myelinated  fibers  representing  the  branches  of  the  axons  of  the  spinal  ganglion 
cells.  Both  types  of  fibers  can  be  followed  through  the  dorsal  roots  into  the 
spinal  cord,  as  well  as  distally  into  the  nerves.  In  the  latter  they  mingle  with 
the  large  myelinated  fibers  coming  from  the  ventral  roots  (Fig.  40).  When 
traced  distally  in  the  peripheral  nerve  the  unmyelinated  fibers  are  found  to  go 
in  large  part  to  the  -kin.  though  a  few  run  in  the  muscular  branches  (Ranson, 
1911  and  1915). 

Classification  of  the  Somatic  Afferent  Fibers  According  to  Function. — 
Sherrington  (1906)  in  an  instructive  book  on  "The  Integrative  Action  of  the 
Nervous  System"  has  furnished  us  with  a  useful  classification  of  the  elements 
belonging  to  the  afferent  side  of  the  nervous  system.  He  designates  those 
carrying  impulses  from  the  viscera  as  interoceptive,  and  subdivides  the  somatic 
afferent  elements  into  exteroceptive  and  proprioceptive  groups.  The  extero- 
ceptive fibers  carry  impulses  from  the  surface  of  the  body  and  from  such  sense 
organs,  as  the  eye  and  ear,  that  are  designed  to  receive  stimuli  from  without. 
These  fibers,  therefore,  are  activated  almost  exclusively  by  external  stimuli. 
The  proprioceptive  fibers,  on  the  other  hand,  respond  to  stimuli  arir-ing  within 
the  bodv  itself  and  convey  impulses  from  the  muscles,  joints,  tendons,  and  the 
semicircular  canals  of  the  ear.  Each  group  has  receptors  or  sensory  endings 
designed  to  respond  to  its  appropriate  set  of  stimuli,  and  for  each  there  are 
special  connections  within  the  brain  and  spinal  cord. 

Exteroceptive  fibers  and  sensory  endings  are  activated  by  changes  in  the 
environment,  that  is  to  say.  they  are  stimulated  by  objects  outside  the  body. 
The  impulses,  produced  in  this  way  and  carried  by  these  fibers  to  the  spinal 
cord,  call  forth  for  the  most  part  reactions  of  the  body  to  its  environment; 
and,  when  relayed  to  the  cerebral  cortex,  they  may  be  accompanied  by  sensa- 
tions of  touch,  heat,  cold,  or  pain.  The  receptors  are,  for  the  most  part,  located 
in  the  skin ;  yet  it  is  convenient  to  include  in  the  exteroceptive  group  the  pressure 
receptors  which  are  closely  allied  to  those  for  touch,  but  which  lie  below  the 
surface  of  the  body.  At  this  point  it  should  be  noted  that  sensibility  to  those 
forms  of  contact  which  include  some  slight  pressure,  such  as  the  placing  of  a 
finger  on  the  skin,  is  not  abolished  by  the  section  of  all  of  the  cutaneous  nerves 
going  to  the  area  in  question,  since  the  deeper  nerves  carry  fibers  capable  of 
responding  to  such  contacts  (Head.  1905).  This  deep  contact  sensibility,  which 
for  lack  of  a  better  name  we  may  call  "pressure-touch,"  must  not  be  overlooked 
in  the  analysis  of  cutaneous  sensations. 

The  balance  of  evidence  is  in  favor  of  the  assumption  that  each  of  the  vari- 


111!     SPINAL    NERVES 


eties  of  cutaneous  sensation  is  mediated  by  a  separate  set  of  nerve  fibers.  Hut 
little  progress  has  as  yet   been  made  toward  identifying  these  various  func- 

tional  groups.  We  know  that  both  myelinated  and  unmyelinated  fibers  are 
present  in  the  cutaneous  aerves  (Ranson,  1915),  hut  arc  not  able  to  say  with 
certainty  which  subserve  each  of  the  varieties  el"  cutaneous  sensation.  There 
arc  many  good  reasons,  however,  for  the  belief  that  painful  afferent  impulses 
and  possibly  also  those  of  temperature  are  carried  by  the  unmyelinated  fibers, 
and  that  those  of  the  touch  and  pressure  group  are  mediated  by  the  myelinated 
libers.  The  evidence  on  which  this  statement  is  based  has  been  briefly  sum- 
marized on  pages  102  104. 


Fig.  41. — Free  nerve  endings  in  the  epidermis  of  a  cat's  paw:  A,  Stratum  corneum;  B,  stratum 
germinativum  Malpighii,  and  C,  its  deepest  portion;  a,  large  nerve  trunk;  b,  collateral  fibers;  c, 
terminal  branches;  d,  terminations  among  the  epithelial  cells.     Golgi  method.     (Cajal.) 

All  sensory  nerve  endings  in  the  skin  belong  to  the  exteroceptive  group, 
but  it  is  not  so  easy  to  say  which  ones  are  responsible  for  each  of  the  several 
varieties  of  cutaneous  sensation,  namely,  touch,  pain,  heat,  and  cold.  On 
structural  grounds  we  may  recognize  three  principal  groups:  (1)  endings  in  hair- 
follicles,  (2)  encapsulated  nerve  endings,  and  (3)  free  terminations  in  the  epi- 
dermis. 

Free  Nerve  Endings. — Some  of  the  myelinated  fibers  as  they  approach 
their  terminations  divide  repeatedly.  At  first  the  branches  retain  their  sheaths, 
but  after  many  divisions  the  myelin  sheaths  and  finally  the  neurilemma  are  lost 
and  only  the  naked  axis-cylinders  remain.     These  enter  the  epidermis,  where, 


68  THE    NERVOUS   SYSTEM 

after  further  divisions,  they  end  among  the  epithelial  cells  (Fig.  41).  This  type 
of  nerve  ending  is  found  in  the  skin,  mucous  membranes,  and  cornea.  Similar 
endings  are  also  found  in  the  serous  membranes  and  intermuscular  connective 
tissue. 

We  do  not  know  what  form  the  endings  of  the  afferent  unmyelinated  fibers 
may  take,  but  it  is  not  unlikely  that  they  also  ramify  in  the  epidermis  like  the 
terminal  branches  of  the  myelinated  fibers  just  described.  It  seems  certain 
that  at  least  a  part  of  the  free  nerve  endings  in  the  epidermis  are  pain  receptors. 
In  the  central  part  of  the  cornea,  the  tympanic  membrane,  and  the  dentine 
and  pulp  of  the  teeth,  such  free  nerve  endings  alone  are  present,  and  pain  is  the 
only  sensation  that  can  be  appreciated. 

Some  of  the  nerve-libers  which  enter  the  epidermis  end  in  disk-like  expansions 
in  contact  with  specialized  epithelial  cells  CFig.  42).     These  have  been  known 


Fig.  42. — Merkel's  corpuscles  or  tactile  disks  from  the  skin  of  the  pig's  snout.  The  nerve- 
fiber,  n,  branches  and  each  division  ends  in  an  expanded  disk,  m,  which  is  attached  to  a  modified 
cell  of  the  epidermis,  a;  c,  an  unmodified  epithelial  cell.      (Ranvier,  Herrick.) 

as  Merkel's  touch-ails  on  the  supposition  that  the  endings  in  question  are  tactile 
receptors. 

Encapsulated  Nerve  Endings. — Among  the  encapsulated  nerve  endings  are 
the  corpuscles  of  Meissner.  These  have  quite  generally  been  regarded  as  tactile 
end  organs  and  are  located  in  the  corium  or  subepidermal  connective  tissue  of 
the  hands  and  feet,  forearm,  lips,  and  certain  other  regions.  They  are  of  large 
size,  oval,  possess  a  thin  connective-tissue  capsule,  and  within  each  terminate 
one  or  more  medullated  fibers  (Fig.  43).  Within  the  capsule  the  fibers  lose  their 
myelin  sheaths,  make  a  variable  number  of  spiral  turns,  and  finally  break  up 
into  many  varicose  branches,  which  form  a  complex  network.  To  another 
type  of  encapsulated  end  organ  belong  those  known  as  the  end  bulbs  of  Krause. 
One  of  these  is  illustrated  in  Fig.  44.  They  are  found  in  the  conjunctiva,  edge 
of  the  cornea,  lips,  and  some  other  localities. 


I  III.    SPINAL    \i  i 


69 


Fig.     43. — -Meissncr's     tactile     corpuscle.  Fig.  44. — End-bulb  of  Krausc  from  con- 

Methylene-blue  stain.     (Dogicl,  Bohm-David-       junctiva     of     man.        Methylene-blue     stain. 
ofF-Huber.)  (Dogiel,  Bohm-Davidoff-Huber.) 


Fig.  45. — Pacinian  corpuscles  from  mesorectum  of  kitten:  A,  Showing  the  fine  branches  of 
the  central  fiber;  B,  the  network  of  fine  nerve-fibers  about  the  central  fiber.  Methylene-blue  stain. 
(Sala,  Bohm-Davidoff-Huber.) 

The  Pacinian  corpuscles,  two  of  which  are  illustrated  in  Fig.  45,  have  a  very 
wide  distribution  in  the  deeper  parts  of  the  dermis  of  the  hands  and  feet,  in  the 


7o 


THE   NERVOUS    SYSTEM 


tendons,  intermuscular  septa,  periosteum,  peritoneum,  pleura,  and  pericardium. 
They  are  also  numerous  in  the  neighborhood  of  the  joints.  According  to  Her- 
rick  (1918)  it  is  probable  that  "by  these  end  organs  relatively  coarse  pressure 
may  be  discriminated  and  localized  (exteroceptive  function),  and  movements 
of  muscles  and  joints  can  be  recognized  (proprioceptive  function)."     They  are 


! —  hst 


Fig.  46. — Nerves  and  nerve  endings  in  the  skin  and  hair-follicles:  list,  Stratum  corneum;  rm, 
stratum  germinativum  Malpighii;  c,  most  superficial  nerve-fiber  plexus  in  the  cutis;  n,  cutaneous 
nerve;  is,  inner  root  sheath  of  hair;  as,  outer  root  sheath;  h,  the  hair  itself;  dr,  glandulae  sebaceae. 
(Retzius,  Barker.) 


large  oval  corpuscles,  made  up  in  great  part  of  concentric  lamella?  of  connective 
tissue.  The  axis  of  the  corpuscle  is  occupied  by  a  core  of  semifluid  substance 
containing  the  termination  of  a  nerve-fiber.  The  fiber  loses  its  myelin  sheath 
as  it  enters  the  core,  through  which  it  passes  from  end  to  end.  Its  terminal 
branches  end  in  irregular  disks.     Side  branches  are  also  given  off  within  the  core. 


Illi:    SPIN  \l.    NERVES 


71 


Nerve  Endings  in  the  Hair-follicles.     It  has  long  been  known  thai  the  hairs 
arc  delicate  tactile  organs.    The  hair-clad  parts  lose  much  of  their  responsive- 


Fig.  47. — Neuromuscular  nerve  end-organ  from  a  dog.  The  figure  shows  the  intrafusal 
muscle-fibers,  the  nerve-fibers  and  their  terminations,  but  not  the  capsule  nor  the  sheath  of  Henle. 
Methylene-blue  stain.     (Huber  and  De  Witt.) 

ness  to  touch  when  the  hair  is  removed.  As  would  be  expected  on  these  ground-. 
the  hair-follicles  are  richly  supplied  with  nerve  endings.  Just  below  the  open- 
ing of  the  sebaceous  glancl  into  the  follicle  mvelinated  nerve-fibers  enter  it,  los- 


72 


THK    NERVOUS    SYS  I  I.M 


ing  their  myelin  sheaths  as  they  enter.  They  give  off  horizontal  branches, 
which  encircle  the  root  of  the  hair,  and  from  these  ascending  branches  arise 
(Fig.  46).  Some  of  these  are  connected  with  leaf-like  expansions,  associated 
with  cells  resembling  Merkel's  touch-cells. 

Practically  nothing  is  known  concerning  the  receptors  for  sensations  of  heat 
and  cold. 

Proprioceptive  Fibers  and  Sensory  Nerve  Endings.—  To  this  group  belong 
the  afferent  elements  which  receive  and  convey  the  impulses  arising  in  the 
muscles,  joints,  and  tendons.  Changes  in  tension  of  muscles  and  tendons  and 
movements  of  the  joints  are  adequate  stimuli  for  the  receptors  of  this  class  and 
excite  nerve  impulses  which,  on  reaching  the  central  nervous  system,  give  in- 
formation concerning  tension  of  the  muscles  and  the  relative  position  of  the 
various  parts  of  the  body.  For  the  most  part,  however,  these  impulses  do  not 
rise  into  consciousness,  but  serve  for  the  subconscious  control  of  muscular 
activitv.  The  unsteady  gait  of  a  tabetic  patient  illustrates  the  lack  of  mus- 
cular control  that  results  when  these  impulses  are  prevented  from  reaching  the 
central  nervous  system. 

The  proprioceptive  fibers  are  myelinated  and  are  associated  with  motor 
fibers  in  the  nerves  to  the  muscles.  Some  follow  along  the  muscles  to  reach 
the  tendons.  Three  types  of  end  organs  belong  to  this  group.  Pacinian  cor- 
puscles, muscle  spindles,  and  neurotendinous  end  organs.  Many  Pacinian 
corpuscles  are  found  in  the  neighborhood  of  the  joints.  They  have  been  de- 
scribed in  a  preceding  paragraph. 

Neuromuscular  End  Organs. — The  afferent  fibers  to  the  muscles  end  on 
small,  spindle-shaped  bundles  of  specialized  muscle-fibers  (Fig.  47).  These 
muscle  spindles  are  invested  by  connective- tissue  capsules;  and  within  each 
of  them  one  or  more  large  myelinated  nerve-fibers  terminate.  Within  the 
spindle  the  myelin  sheath  is  lost  and  the  branches  of  the  axis-cylinders  wind 
spirally  about  the  specialized  muscle-fibers,  or  they  may  end  in  irregular  disks. 
Somewhat  analogous  structures  are  the  neurotendinous  end  organs  or  tendon 
spindles  where  myelinated  nerve-fibers  end  in  relation  to  specialized  tendon 
fasciculi. 


CHAPTER  V] 

THE  SPINAL  CORD 

The  spinal  conl.  or  medulla  spinalis,  is  a  cylindric  mass  of  nervous  tissue 
occupying  the  vertebral  canal.  It  is  40  to  45  cm.  in  Length,  reaching  from  the 
foramen  magnum,  where  it  is  continuous  with  the  medulla  oblongata,  to  the 
level  of  the  first  or  second  lumbar  vertebra.     Even  above  this  level  the  vertebral 

canal  is  by  no  means  fully  occupied  by  the  cord  (Fig.  48),  which,  as  shown  in 
Fig'  49,  is  surrounded  by  protective  membranes,  while  between  these  and  the 
wall  of  the  canal  is  a  rather  thick  cushion  of  adipose  tissue  containing  a  plexus 


Extradural  fat  and  venous  plexus      m    spinal  cord 

Subarachnoid  space  \  J   |l     /  Cura  mater 

S [) nuil  nerve  roots  XY     •\\J /      Ligamentum  denticulatum 


Fig.  48. — Diagram  showing  the  relation  of  the  spinal  cord  to  the  vertebral  column. 

of  veins.  Immediately  surrounding  the  cord  and  adherent  to  it  is  the  delicate, 
highly  vascular  pia  mater.  This  is  separated  from  the  thick,  fibrous  dura  mater 
by  a  membrane  having  the  tenuity  of  a  spider  web,  the  arachnoid,  which  sur- 
rounds the  subarachnoid  space.  This  space  is  broken  up  by  subarachnoid 
trabecular  and  filled  with  cerebrospinal  fluid. 

External  Form. — The  spinal  cord  is  not  a  perfect  cylinder,  but  is  somewhat 
flattened  ventrodorsally.  especially  in  the  cervical  region.  Its  diameter  is  not 
uniform  throughout,  being  less  in  the  thoracic  than  in  the  cervical  and  lumbar 
portions.  That  is  to  say,  the  cord  presents  two  swellings  (Fig.  51).  The  cer- 
vical enlargement  (intumescentia  cervicalis)  comprises  all  that  portion  of  the  cord 

73 


74 


THE   NERVOUS    SYSTEM 


from  which  the  nerves  of  the  brachial  plexus  arise,  that  is.  the  fourth  cervical 
to  the  second  thoracic  segments  inclusive.  The  lumbar  enlargement  (intumes- 
centia  lumbalis)  is  not  quite  so  extensive  and  corresponds  less  accurately  to  the 
origin  of  the  nerves  innervating  the  lower  extremity.  At  an  early  stage  in  the 
embryonic  development  of  the  spinal  cord  these  enlargements  are  not  present. 
In  the  time  of  their  first  appearance  and  in  their  subsequent  growth  they  are 
directly  related  to  the  development  of  the  limbs. 

Below  the  lumbar  enlargement  the  spinal  cord  rapidly  decreases  in  size 
and  has  a  cone-shaped  termination,  the  conns  mcdullaris,  from  the  end  of  which 
a  slender  filament,  the  filum  tcrminalc.  is  prolonged  to  the  posterior  surface  of 
the  coccyx  (Figs.  50.  51).  This  terminal  filament  descends  in  the  middle  line. 
surrounded  by  the  roots  of  the  lumbar  and  sacral  nerves,  to  the  caudal  end  of 

Septum  posiicum 

Posterior  spinal  artery 

Ligamentum  dentkulatum 
Subarachnoid  trabecule ----- 


Pia  mater  - 


Epidural  trabecule 

Anterior  spinal  artery 


—  Dura  mater 
*~«  Subdural  space 
A  ra  ditto  id 


'-Nerve  root 
Subarachnoid  cavity 


Linca  splendetis 


Fig.  49. — Diagram  of  the  spinal  cord  and  meninges. 

the  dural  sac  at  the  level  of  the  second  sacral  vertebra.  Here  it  perforates  the 
dura  mater,  from  which  it  receives  an  investment,  and  then  continues  to  the 
posterior  surface  of  the  coccyx.  The  last  portion  of  the  filament  with  its  dural 
investment  is  often  called  the  filum  of  the  spinal  dura  mater  (filum  durae  matris 
spinalis).  The  filum  terminale  is  composed  chiefly  of  pia  mater;  but  in  its 
rostral  part  it  contains  a  prolongation  of  the  central  canal  of  the  cord. 

The  spinal  cord  shows  an  obscure  segmentation,  in  that  it  gives  origin  to 
thirty-one  pairs  of  metameric  nerves.  These  segments  may  be  somewhat 
arbitrarily  marked  ofl  from  each  other  by  passing  imaginary  planes  through  the 
highest  root  filaments  of  each  successive  spinal  nerve  (Donaldson  and  Davis. 
1903).  The  highest  of  these  planes,  being  just  above  the  origin  of  the  first  cer- 
vical nerve,  marks  the  separation  of  the  spinal  cord  from  the  medulla  oblongata. 


THE    SPIN  \i.    <()KD 


75 


This  is  again  an  arbitrary  line  of  separation,  since  both  as  to  external  form 
and  Interna]  structure  the  cord  passes  over  into  the  medulla  oblongata  by  in 


Medulla  oblongata 


v-T*T7"  .V.  cervicalis  VIII 

A 

i:J —  Ventral  root  of  X. 
T.  Ill 

Dorsal  root  of  X . 
T.  IV 

-  Lateral   funiculus 
Spinal  dura  mater 


N.  thoricalis  XII 


r  Pom 

Medulla  oblongata. 


--Anterior  median  fissure 
—Anterolalt  ral  sulcus 
-( 'ervical  enlargement 

-Anterior  funiculus 


-Thoracic  portion  of- 
spinal  cord 


Lumbar  enlargement 


^> 


Rhomboid ; 


I'n  tiritir   median 

Mill  US 

P     •  rior  funic- 
ulus 

Posterior       inter- 
mediate suit  us 

Dorsal  root 


A 


Cauda  equina 


o^JZ-  N.  lumbal  is  V 


■  ,■ Filum  of  spinal  dura 

mater 


.Con  us  medullar  is 


.-Filum  lerminale 


—  Cauda  equina 


Fig.  50.  Fig.  51.  Fig.  52. 

Figs.  50-52. — Three  views  of  the  spinal  cord  and  rhombencephalon:  Fig.  50,  Lateral  view 
with  spinal  nerves  attached;  Fig.  51,  ventral  view  with  spinal  nerves  removed;  Fig.  52,  dorsal 
view  with  spinal  nerves  attached.      (Modified  from  Spalteholz.) 

sensible  gradations.  According  to  this  method  of  subdivision  there  are  in  the 
cervical  portion  of  the  cord  eight  segments,  in  the  thoracic  twelve,  in  the  lumbar 
five,  and  in  the  sacral  five,  while  there  is  but  one  coccygeal  segment. 


76  THE    NERVOUS   SYSTEM 

Several  longitudinal  furrows  are  seen  upon  the  surface  of  the  cord  (Figs.  51, 
52).  Along  the  middle  line  of  the  ventral  surface  is  the  deep  anteriniL median 
fissure  (fissura  mediana  anterior).  This  extends  into  the  cord  to  a  depth 
amounting  to  nearly  one-third  of  its  anteroposterior  diameter  and  contains  a 
fold  of  pia  mater.  Along  the  middle  line  of  the  dorsal  surface  there  is  a  shallow 
groove,  the  posterior  median  salens  (sulcus  medianus  posterior; .  As  may  be 
i en  in  cross-sections  of  the  spinal  cord,  it  i-.  divided  into  approximately  sym- 
metric lateral  halves  by  the  two  furrows  just  described  and  by  the  posterior 
median  septum  (Figs.  55,  56,  57).  On  either  side,  corresponding  to  the  line  of 
origin  of  the  ventral  roots,  i.-^  a  broad,  shallow,  almost  invisible  groove,  the 
anterolateral  sulcus  (sulcus  lateralis  anterior).  And  again  on  either  side,  cor- 
responding to  the  line  of  origin  of  the  dorsal  roots,  is  the  narrower  but  deeper 
posterolateral  sulcus  (sulcus  lateralis  posterior).  These  six  furrows  extend  the 
entire  length  of  the  spinal  cord.  In  the  cervical  region  an  additional  longi- 
tudinal groove  may  be  seen  on  the  dorsal  surface  between  the  posterior  median 
and  posterolateral  sulci,  but  somewhat  nearer  the  former.  It  is  known  as  the 
posterior  intermediate  sulcus  and  extends  into  the  thoracic  cord,  where  it  grad- 
ually disappears. 

Funiculi.  By  means  of  these  furrows  and  the  subjacent  gray  matter  each 
lateral  half  of  the  cord  is  subdivided  into  columns  of  longitudinally  coursing 
nerve-fibers  known  as  the  anterior,  lateral,  and  posterior  funiculi  (funiculus 
.interior,  funiculus  lateralis  et  funiculus  posterior).  In  the  cervical  and  upper 
thoracic  regions  the  posterior  intermediate  sulcus  divides  the  posterior  funiculus 
into  a  medial  portion,  the  fasciculus  gracilis,  and  a  lateral  portion,  the  fasciculus 
cuneatus. 

Nerve  Roots. — From  the  lateral  funiculus  in  the  upper  four  to  six  cervical 
segments  there  emerge,  a  little  in  front  of  the  dorsal  roots  of  the  spinal  nerves, 
a  series  of  root  filaments  which  unite  to  form  the  spinal  root  of  the  accessory 
nerve  (Fig.  125).  This  small  nerve  trunk  ascends  along  the  side  of  the  cord, 
enters  the  cranial  cavity  through  the  foramen  magnum,  and  carries  to  the 
accessory  nerve  the  fibers  for  the  innervation  of  the  sternocleidomastoid  and 
trapezius  muscles. 

From  the  posterolateral  sulcus  throughout  the  entire  length  of  the  spina] 
cord  emerge  an  almost  uninterrupted  series  of  root  filaments  (lila  radicularia). 
Those  from  a  given  segment  of  the  cord  unite  to  form  the  dorsal  root  of  the  cor- 
responding spinal  nerve.  The  filaments  of  the  ventral  roots  emerge  from  the 
broad,  indistinct  anterolateral  sulcus  in  groups,  several  appearing  side  by  side, 


T11K    SPIN  a   CI  IRD 


77 


rather  than  in  the  accurate  linear  order  characteristic  of  the  dorsal  roots.  1  hose 
[null  a  given  segment  unite  with  each  other  to  form  a  ventral  rout;  and  that  in 
turn  joins  with  the  corresponding  dorsal  root  just  beyond  the  spinal  ganglion  to 
form  the  mixed  nerve  I  l'"i,ur.  50). 

Relation  of  the  Spinal  Cord  and  Nerve  Roots  to  the  Vertebral  Column. 
At  an  early  fetal  stage  the  spinal  cord  occupies  the  entire  length  of  the  vertebral 


Infrahyoid  must  les 
Diaphragm 


Muscles  of  shoulder,  arm, 
and  hand 


Abdominal  musdes 


Flexors  of  hip 

Extensors   of  the    knee 

and  adductors  of  hip 


Other    muscles    of   thigh, 
.  and  foot 

Perineal  and  anal  mus- 
cles 


Cervical  segments  of  spinal  cord 


Thoracic  segments  of  spinal  i  ord 


Lumbar  segments  of  spinal  cord 

Sacral  and  coccygeal  segim  i 

spinal  cord 


Fig.  53.— Diagram  showing  the  level  of  the  various  segments  of  the  spinal  cord  with  reference  to 
the  vertebrae,\vith  a  table  showing  the  distribution  of  the  fibers  of  the  several  ventral  roots. 

canal  and  the  spinal  nerves  pass  horizontally  lateralward  to  their  exit  through 
the  intervertebral  foramina.  As  development  progresses  the  vertebral  column 
increases  in  length  more  rapidly  than  the  spinal  cord,  which,  being  firmly  an- 
chored above  by  its  attachment  to  the  brain,  is  drawn  upward  along  the  canal, 
until  in  the  adult  it  ends  at  about  the  lower  border  of  the  first  lumbar  vertebra. 


78  THE    NERVOUS    SYSTEM 

At  the  same  time  the  roots  of  the  lumbar  and  sacral  nerves  become  greatly 
elongated.  They  run  in  a  caudal  direction  from  their  origin  to  the  same  inter- 
vertebral foramina  through  which  they  made  their  exit  before  the  cord  shifted 
its  position.  Since  the  thoracic  portion  of  the  cord  has  changed  its  relative 
position  but  little,  and  the  cervical  part  even  less,  the  cervical  roots  run  almost 
directly  lateralward,  while  those  of  the  thoracic  nerves  incline  but  little  in  a 
caudal  direction. 

Since  the  spinal  cord  ends  opposite  the  first  or  second  lumbar  vertebra,  the 
roots  of  the  lumbar,  sacral,  and  coccygeal  nerves,  in  order  to  reach  their  proper 
intervertebral  foramina,  descend  vertically  in  the  canal  around  the  conus  medul- 
laris  and  filum  terminale.  In  this  way  there  is  formed  a  large  bundle,  which  is 
composed  of  the  roots  of  all  the  spinal  nerves  below  the  first  lumbar  and  has 
been  given  the  very  descriptive  name  cauda  equina. 

The  amount  of  relative  shortening  of  the  various  segments  of  the  cord  differs 
in  different  individuals.  In  Fig.  53,  where  the  quadrilateral  areas  represent  the 
bodies  of  the  vertebrae,  we  have  indicated  the  average  position  of  each  segment 
of  the  spinal  cord.  This  figure  is  based  on  data  published  by  Reid  (1889).  It  is 
obvious  that  the  segments  are  longer  in  the  thoracic  than  in  the  cervical  and 
lumbar  portions  of  the  cord,  while  the  sacral  segments  are  even  shorter  (see 
also  Fig.  59). 

We  have  been  at  some  pains  to  explain  the  development  of  the  cauda  equina 
and  the  vertebral  level  of  the  various  segments  of  the  spinal  cord  because  these 
are  matters  of  much  practical  importance.  In  spinal  puncture  the  needle  is 
made  to  enter  the  subdural  space  caudal  to  the  termination  of  the  cord.  In 
locating  lesions  of  the  spinal  cord  it  is  necessary  to  know  the  position  of  its 
various  segments  with  reference  to  the  vertebrae.  It  is  particularly  important 
to  be  able  to  distinguish  between  an  injury  to  the  lower  part  of  the  spinal  cord 
and  one  which  involves  only  the  nerve  roots  in  the  cauda  equina,  since,  although 
the  symptoms  in  the  two  cases  may  be  nearly  identical,  damage  to  the  spinal 
cord  is  irreparable,  while  the  nerve  roots  will  regenerate. 

The  Spinal  Cord  in  Section. — When  a  section  is  made  through  any  part  of 
the  brain  or  spinal  cord  one  sees  at  once  that  they  are  composed  of  two  kinds 
of  tissue — the  one  whitish  in  color,  the  other  gray,  tinged  with  pink.  The  white 
substance  consists  chiefly  of  myelinated  fibers,  the  gray  is  made  up  of  nerve- 
cells,  dendrites,  unmyelinated  and  myelinated  fibers,  and  many  blood-vessels. 
Both  have  a  supporting  framework  of  neuroglia. 

The  gray  substance  (substantia  grisea)  of  the  spinal  cord  is  centrally  placed 


THE    SPIN  \i.   couij 


79 


and  forms  a  continuous  tinted  column,  which  is  everywhere  enclosed  bj   the 
white  matter  (Fig.  54).     In  cross-section  it  has  the  form  of  a  letter  II  (Fig.  55). 
rhere  is  a   comma-shaped   gray  field    in   each    lateral 
half   of   the   cord,  and    these   are   united    across   the 
middle  line  by  a  transverse  gray  bar.    The   enlarged 
anterior  end  of  the  comma   has  been  known  as  the  Neu- 
tral  horn,   the  tapering   posterior  end    as    the    dorsal 
horn,  and   the  transverse  bar  as  the  gray  commissure. 
But,  when  it  is  remembered  that  the  gray  substance 
forms  a  .continuous  mass  throughout  the  length  of  the 
spinal    cord,   it  will  be  seen  that  the  term   "column" 
is  more  appropriate  than  "horn."     The  long  gray  mass 
in  either  lateral  half  of  the  cord  is  convex  medially  and  concave  laterally.     It 
projects  in  a  dorsolateral  direction  as  the  posterior  column  (columna  posterior). 

As  seen  in  a  cross-section  of  the  cervical  cord,  the  posterior  column  is  rela- 
tively long  and  narrow  and  nearly  reaches  the  dorsolateral  sulcus  (Fig.  55). 


Fig.  54. — Diagram  of  gray 
columns  of  spinal  cord. 


Posterior  intermediate  sulcus  and  septum      Posterior  median  sulcus  and  septum 

Fasciculus  gracilis  ]  p    ,    • 

I    Fasciculus  cuneatus  j  funiculus 


Collaterals  from  cuneale  fuse. 
Substantia  gelatinosa 


Posterolateral  sulcus 
\ 


Posterior  column [  ~ '>ex. ' 
Cervix 

Reticular  formation 


Posterior  ...£* 

com. 
Anterior  ._iisi 
gray  com. 

A  nterioi "7-- 

white  com. 
A  nterior' 
column 

Ventral   root  fibers  ...:_^i_ 
Anterolateral    sulcus 


Dorsal  root 

Dorsolateral  fasciculus 
( Lissauer) 
Lateral  funiculus 


tjGsfe 


^ 


Hi 


m 


'  Central  canal 
Anterior  funiculus 
Anterior  median  fissure 

Fig.  55. — Section  through  seventh  cervical  segment  of  the  spinal  cord  of  a  child.     Pal-Weigert 

method. 


It  presents  a  constricted  portion  known  as  the  cervix,  a  pointed  dorsal  extrem- 
ity or  apex,  and  between  the  two  an  expanded  part  sometimes  called  the  caput. 
The  apex  consists  largely  of  a  special  variety  of  gray  substance,  gelatinous  in 


80  THE    NERVOUS    SYSTEM 

appearance  in  the  fresh  condition  and  very  difficult  to  stain  by  neurologic  meth- 
ods, which  in  sections  has  a  A -shaped  outline.  It  is  known  as  the  substantia 
gelatinosa  Rolandi.  In  the  thoracic  portion  the  posterior  column,  which  is  here 
very  slender,  does  not  come  so  close  to  the  surface;  and  in  the  lumbosacral  seg- 
ments it  is  much  thicker  (Figs.  56,  57). 

The  anterior  column  is  relatively  short  and  thick  and  projects  toward  the 
anterolateral  sulcus.  It  contains  the  cells  of  origin  of  the  fibers  of  the  ventral 
root.  From  its  lateral  aspect  nearly  opposite  the  gray  commissure  there  pro- 
jects a  triangular  mass,  known  as  the  lateral  column  (columna  lateralis).  This 
is  prominent  in  the  thoracic  and  upper  cervical  segments;  but  it  blends  with 
the  expanded  anterior  column  in  the  cervical  and  lumbar  enlargements  (Fig.  56). 

Posterior  median  sulcus  and  septum     Posterior  funiculus 

Substantia  gelatinosa  |     ;  Dorsolateral  fasciculus  (Lissauer) 

Posterolateral  sulcus  /.    (  ,  Dorsal  root 

....  I  ■/'.>        Lateral  funiculus 

Apex  of  posterior  column  -      ,  >?        i 

Nucleus  dorsalis  \      j  /       '  i-.-,. 

Lateral  column '.     -.:^vi*' ~v~\^i-\  •''  v-,*Sk 


Posterior  commissure''' ^ 

Anterior  'white  commissure  " 

Anterior  column' 


'  j' ':.  ■  )'■■'-<*  ~  Central  canal 

■  ''   .  '  -V-~,l  ^'Anterior  funiculus 


'-  Anterior  median  fissure 

Fig.  56. — Section  through  the  seventh  thoracic  segment  of  the  spinal  cord  of  a  child.     Pal-Weigert 

method. 

The  reticular  formation  (formatio  reticularis),  situated  just  lateral  to  the  cer- 
vix of  the  posterior  column  in  the  cervical  region,  is  a  mixture  of  gray  and 
white  matter  (Fig.  55).  Here  a  network  of  gray  matter  extends  into  the  white 
substance,  breaking  it  up  into  fine  bundles  of  longitudinal  fibers.  The  reticular 
formation  is  most  evident  in  the  cervical  region,  but  traces  of  it  appear  at  other 
levels. 

The  gray  commissure  contains  the  central  canal,  and  by  it  is  divided  into  the 
posterior  commissure  (commissura  posterior)  and  the  anterior  gray  commissure 
(commissura  anterior  grisea).  Ventral  to  the  latter  many  medulla  ted  fibers 
cross  the  midline,  constituting  the  anterior  white  commissure. 

The  cavity  of  the  neural  tube  persists  as  the  central  canal,  which  lies  in  the 
gray  commissure  throughout  the  entire  length  of  the  cord.  The  canal  is  so 
small  as  to  be  barely  visible  to  the  naked  eye.     It  is  lined  with  ependymal 


I  ill     SPINAL   ( dkl) 


8l 


epithelium  and  the  lumen  is  often  blocked  with  epithelial  debris.  The  canal, 
which  is  narrowest  in  the  thoracic  region,  expands  within  the  lower  pari  of  the 
eonus  medullaris  to  form  a  fusiform  dilatation,  the  ventriculus  terminalis. 


Posterior  median  sulcus  and  septum 
Collaterals  from  fast  it  ulus  cuneatus     \     Posterior  funiculus 


Dorsal  root 


Substantia  eelatinosa ' 


• 


Posterior  column    ]■'■ 
(  irvix 


Dorsolateral  fasciculus  (Lissauei 
Posterolateral  mU  us 


Lateral  funiculus 


Posterior  commissure  ----^J**** 

Anterior    urav  ----:■"""" 
commissure        llmm 

Anterior  icliile  com..---  •..-. 
Anterior  column f 


' 


Ventral  root  fibers  _ 
Anterolateral  side, 


Central  canal 


\ 


A  nterior  median  fissure 
'Anterior  funiculus 

Fig.  57. — Section  through  the  fifth  lumbar  segment  of  the  spinal  cord  of  a  child.     Pal-Weigerl 

method. 

Dorsal  roots  of  lumbar  and  sacral  nerves 
,/'/      Posterior  funiculus 


Substantia  gelatinosa 
Dorsolateral  fasciculus 

Posterior  column 

,   Lateral  funiculus 


ii 

;■:.■■■* 


Anterior  column 

Ventral  roots  of  lumbar  and 
sacral  nerves 


Fig.  58.— Section  of  the  third  sacral  segment  of  the  human  spinal  cord  and  the  lumbosacral  nerve 
roots  of  the  Cauda  ecpiina.      Pal-Weigert  method. 

The  White  Substance.— The  long  myelinated  fibers  of  the  cord,  arranged  in 
parallel  longitudinal  bundles,   constitute  the  white  substance  which   forms  a 


82 


THE    NERVOUS    SYSTEM 


thick  mantle  surrounding  the  gray  columns.     In  each  lateral  half  of  the  cord  it 
is  divided  into  the  three  great  .strands  or  funiculi,  which  have  been  described 


While  matter. 

Grey  matter. 

— Enhire  secrion. 

100 

• 

ao 

-"""^ .-'..     \ 

• 

60 

(l 

.— "^        \ 

40 

20 

-.\    \ 

I  11  III  IY  Y  \1  YUYin  1     II    III 

IY 

Y 

YJ 

YII      YJ1I      IX 

X 

XI     XII 

1     D  III  IVY  I  IlUJKVl 

Fig.  59. — Curves  showing  the  variations  in  sectional  area  of  the  gray  matter,  the  white  matter,  and 
the  entire  cord  in  the  various  segments  of  the  human  spinal  cord.     (Donaldson  and  Davis.) 

on  the  surface  of  the  cord.  The  anterior  funiculus  ('funiculus  anterior)  is  bounded 
by  the  anterior  median  fissure,  the  anterior  column,  and  the  emergent  fibers 
of  the  ventral  roots.     The  lateral  funiculus  (funiculus  lateralis)  lies  lateral  to 


VII  c-  VI It  c 


VI1IC -ID 


II  D 


VII  D 


IV  S  C 

Fig.  60. — Outline  drawings  of  sections  through  representative  segments  of  the  human  spinal  cord. 

the  gray  substance  between  the  anterolateral  and  posterolateral  sulci,  i.  c., 
between  the  lines  of  exit  of  the  ventral  and  dorsal  roots.  The  posterior  funiculus 
(funiculus  posterior)  is  bounded  by  the  posterolateral  sulcus  and  posterior  col- 


I  in:    SPINAL   '  i  U'i» 

ChARACTI  RISTN    I  I   Ml  RES  01   TRANSVERSI    Se<  i  m.n-  \  I   \  \i;imi  -  I  i  mi      .  i]    mi    SPINA!    I   ORD 


Level 

i  m  ii. il. 

1  Imrucic. 

1  umliar. 

( )ui  line 

( )val,    greatest    di- 
ameter  transverse 

( )\ ,il  to  cir<  nI. ii 

Nearly  i  irculai 

(   in  ul. ii  to 

quadrilateral 

Volume  «>i  graj 
matter 

1  arge 

Sim. ill 

Relatively 
large 

Antei  i,,r  graj 
column 

Massive 

Slender 

Massive 

Massive 

Posterior  graj 
column 

Relatively  slender, 

but  extends  far 
posteriorly 

Slender 

Massive 

Ma-sue 

Lateral  gray 

column 

Absorbed  in  the 
anterior  except  in 
tin'  upper  three 
cervical  segments 

Well  marked 

Absorbed  in  the 

anterior  column 

Present 

Processus 

reticularis 

Well  developed 

Poorly  developed 

Absent 

Absent 

White  matter 

In  large  amount 

Less  than  in  the 
cervical  region, 
but  relatively  a 
large  amount  in 
comparison  to  the 
gray  matter 

Slightly  less  than  in 
the    thoracic    re- 
gion;   very    little 
in  comparison  to 
the  large  volume 
of  the  gray 

Very  little 

Sulcus  interme- 
dius  posterior 

Present  throughout 

Present  in  upper 
seven  thoracic 
segments 

Absent 

Absent 

uran  on  the  one  side,  and  the  posterior  median  septum  on  the  other.  The  sep- 
tum, just  mentioned,  completely  separates  the  two  posterior  funiculi  from  each 
other.  Incomplete  septa  project  into  the  white  substance  from  the  enveloping 
pia  mater.  One  of  these,  more  regular  than  the  others,  enters  along  the  line  of 
the  posterior  intermediate  sulcus.  It  is  restricted  to  the  cervical  and  upper 
thoracic  segments,  is  known  as  the  posterior  intermediate  septum,  and  divides 
the  posterior  funiculus  into  two  bundles,  the  more  medial  of  which  is  known 
as  the  fasciculus  gracilis,  while  the  other  is  called  the  fasciculus  cuncatus. 

Characteristics  of  the  Several  Regions  of  the  Spinal  Cord  — It  will  be  ap- 
parent from  Figs.  55-58  that  the  size  and  shape  of  the  spinal  cord,  as  seen  in 
transverse  section,  varies  greatly  at  the  different  levels  and  that  the  relative 
proportion  of  gray  and  white  matter  is  equally  variable.      Two  factors   are 


84  THE    NERVOUS    SYSTEM 

primarily  responsible  for  these  differences.  One  of  these  is  the  variation  in  the 
size  of  the  nerve  roots  at  the  different  levels;  for  where  great  numbers  of  nerve- 
fibers  enter,  they  cause  an  increase  in  the  size  of  the  cord  and  particularly  in 
the  volume  of  the  gray  matter.  It  has  already  been  pointed  out  that  the  cer- 
vical and  lumbar  enlargements  are  directly  related  to  the  large  nerves  supply- 
ing the  extremities.  The  second  factor  is  this:  Since  all  levels  of  the  cord  are 
associated  with  the  brain  by  bundles  of  long  fibers,  it  is  obvious  that  such  long 
fibers  must  increase  in  number  and  the  white  matter  increase  in  volume  as  we 
follow  the  cord  from  its  caudal  end  toward  the  brain.  All  this  is  well  illus- 
trated in  a  diagram  by  Donaldson  and  Davis  reproduced  in  Fig.  59. 

The  outline  of  a  section  of  the  spinal  cord  at  the  fourth  sacral  segment  is  some- 
what quadrilateral.  The  total  area  is  small  and  the  greater  part  is  occupied 
by  the  thick  gray  columns  (Fig.  60).  The  size  of  the  cord  is  much  greater  at 
the  level  of  tine  first  sacral  and  fifth  lumbar  segments,  as  might  be  expected  from 
the  large  size  of  the  associated  nerves  (Figs.  57,  60).  There  is  both  an  absolute 
and  a  relative  increase  in  the  white  substance,  which  here  contains  the  long 
paths  connecting  the  sacral  portions  of  the  spinal  cord  with  the  brain.  Both 
the  anterior  and  posterior  columns  are  massive,  and  the  anterior  presents  a 
prominent  lateral  angle.  The  large  nerve-cells  in  the  lateral  part  of  the  an- 
terior column  give  rise  to  the  fibers  which  run  to  the  muscles  of  the  leg.  At  the 
level  of  the  seventh  thoracic  segment  (Figs.  56.  60)  the  cross-sectional  area  is  less 
than  in  the  lumbar  enlargement.  Corresponding  to  the  small  size  of  the  tho- 
racic nerves  the  gray  matter  in  this  region  is  much  reduced,  both  anterior  and 
posterior  columns  being  very  slender.  The  apex  of  the  latter  is  some  distance 
from  the  surface  and  its  cervix  is  thickened  by  a  column  of  cells  known  as  the 
nucleus  dorsalis.  The  columna  lateralis  is  prominent.  The  white  matter  is 
somewhat  more  abundant  than  in  the  lumbar  region,  and  increases  slightly  in 
amount  as  we  follow  the  cord  rostrally  through  the  thoracic  region  (Fig.  59). 

A  transverse  section  at  the  level  of  the  seventh  cervical  segment  is  elliptic  in 
outline  and  has  an  area  greater  than  that  of  any  other  level  of  the  cord  (Figs. 
55,  60).  The  white  matter  is  voluminous  and  contains  the  long  fiber  tracts 
connecting  the  brain  with  the  more  caudal  portions  of  the  cord.  The  gray 
matter  is  also  abundant,  as  we  might  expect  from  the  large  size  of  the  seventh 
cervical  nerve.  The  ventral  column  is  especially  thick  and  presents  a  prominent 
lateral  angle.  The  large  laterally  placed  nerve-cells  of  the  anterior  column  are 
associated  with  the  innervation  of  the  musculature  of  the  arm.  The  posterior 
column  is  relativelv  slender,  but  reaches  nearlv  to  the  dorsolateral  sulcus. 


I  III     SPINAL    <  <)ki) 


85 


MICROSCOPIC  ANATOMY 

Neuroglia.  Occupying  the  Interstices  among  the  true  nervous  elements  of 
the  centra]  nervous  system  is  a  peculiar  supporting  tissue,  the  neuroglia,  which 
is  of  ectodermal  origin.  In  the  chapter  on  Histogenesis  we  Learned  thai  from 
the  original  epithelium  of  the  neural  tube  there  are  differentiated  spongioblasts 
and  neuroblasts,  as  well  as  a  special  epithelial  lining  for  the  tube,  the  ependyma. 


Fig.  61. — Ependyma  and  neuroglia  in  the  region  of  the  central  canal  of  a  child's  spinal  cord: 
A,  Ependymal  cells;  B  and  D,  spider  cells  in  the  white  and  gray  matter,  respectively;  C,  mossy 
cells.     Golgi  method.     (Cajal.) 

The  latter  consists  of  long  nucleated  columnar  cells  which  line  the  central  canal 
of  the  spinal  cord  as  well  as  the  ventricles  of  the  brain  (Fig.  61).  In  fetal  life 
their  free  ends  bear  cilia,  which  project  into  the  lumen  of  the  tube,  and  fine 
processes  from  the  outer  ends  extend  to  the  periphery  of  the  cord.  In  the  adult 
there  are  no  cilia  and  the  peripheral  processes  reach  the  surface  only  along  the 
posterior  median  septum  and  in  the  anterior  median  fissure. 


86  THE   NERVOUS    S  VST  EM 

The  neuroglia  cells  are  differentiated  from  the  spongioblasts.  These,  when 
stained  by  the  Golgi  method,  appear  as  small  cells  with  many  processes.  Some 
have  long  slender  processes,  the  spider  cells  or  long  rayed  astrocytes;  others  have 
short  thick  varicose  processes,  the  mossy  cells  or  short  rayed  astrocytes  (Tig. 
61).  Special  neuroglia  stains,  like  that  of  Weigert,  show  that  an  astrocyte  is 
composed  of  a  glia  cell  associated  with  many  glia  fibers.  Some  authors  main- 
tain that  the  fibers  run  through  the  cytoplasm,  while  others  assert  that  they 
merely  pass  along  the  surface  of  the  cell.  In  any  case  the  fibers  are  to  be  re- 
garded as  products  of  these  cells.  Neuroglia  cells  and  fibers  are  found  every- 
where throughout  the  gray  and  white  matter  of  the  spinal  cord,  forming  a  sup- 
porting framework  for  the  nervous  elements.     A  special  condensation  of  neu- 


f'/i  % 

;--      :{-: — Unmyelinated  fibers 

"   "■      - — ! — Myelinated  fibers 

■•■■■     '.       ;    ,-■'''     ..     '      ■     : 

Fig.  62. — From  a  cross-section  through  the  spinal  cord  of  a  rabbit  showing  the  structure  of  the  white 
matter  as  revealed  by  the  Cajal  method.     (Cajal.) 

roglia  surrounds  the  central  canal  and  is  known  as  the  substantia  gelatinosa 
centralis.  In  addition  to  the  neuroglia  this  contains  some  nerve-fibers  and 
cells.  Beneath  the  pia  mater  and  closely  investing  the  spinal  cord  externally 
is  a  thin  stratum  of  neuroglia,  the  glial  sheath,  which  dips  into  the  cord  along 
with  the  pial  septa.  The  posterior  median  septum  is  composed  of  neuroglia 
and  greatly  elongated  ependymal  elements,  and  is  in  no  part  formed  by  the 
pia  mater. 

White  Substance. — The  white  matter  of  the  spinal  cord  consists  of  longi- 
tudinally coursing  bundles  of  nerve-fibers,  bound  together  by  a  feltwork  of 
neuroglia  fibers  in  which  are  scattered  neuroglia  cells.  A  majority  of  the  neu- 
roglia fibers  run  in  a  direction  transverse  to  the  long  axis  of  the  nerve-fibers. 
Blood-vessels  enter  the  cord  from  the  pia  mater  and  are  accompanied  by  con- 


THE    SPINAL    (OKI) 


87 


nective  tissue  from  the  pia  and  by  the  subpial  aeuroglia.  It  has  been  generally 
supposed  that  the  white  Fascicles  of  the  cord  were  composed  almosl  exclusively 
of  myelinated  Gibers;  and  it  is  true  that  these,  parti)  because  of  their  size,  are 
the  most  conspicuous  elements.  In  cross-sections  stained  In  the  Weigert 
method  the  myelin  sheaths  alone  are  stained;  and  since  the  fibers  are  cut  at 
right  angles  to  their  long  axes,  they  appear  as  rings.  Cajal  (1009)  has  shown 
that  there  are  also  great  numbers  of  unmyelinated  fibers  in  the  longitudinal 
fascicles  of  the  cord  (Fig.  62).  The  different  Fascicles  differ  not  only  in  the  size 
of  their  myelinated  fibers  but  also  in  the  proportion  of  unmyelinated  fibers 
which  they  contain.  The  fasciculus  dorsolaterals  or  tract  of  Lissauer  (Fig.  63) 
contains   fine   myelinated   fibers   with   great   numbers  of   unmyelinated   axons. 


Fig.  63. — From  a  cross-section  of  the  spinal  cord  of  the  cat;  a  narrow  strip  extending  across 
the  apex  of  the  posterior  gray  column:  a,  Fasciculus  cuneatus;  b,  fasciculus  dorsolateralis  (Lis- 
sauer); c,  dorsal  spinocerebellar  tract.  The  unmyelinated  fibers  appear  as  black  dots.  Pyridin- 
silver  method. 

Close  to  it  lies  the  dorsal  spinocerebellar  tract  which  is  composed  almost  ex- 
clusively of  large  myelinated  fibers. 

Gray  Substance. — The  gray  matter  is  composed  of  nerve-cells,  including 
their  dendrites,  and  of  unmyelinated  axons  and  smaller  numbers  of  myelinated 
fibers — all  supported  by  a  neuroglia  framework  and  richly  supplied  with  capil- 
lary blood-vessels.  The  axons  of  the  cells  of  Golgi's  Type  I  are  very  long  and 
run  out  into  the  white  substance  or  into  the  ventral  roots.  Those  of  the  cells 
of  his  Type  II  are  short  and  end  within  the  gray  matter.  In  addition,  great 
numbers  of  collaterals  from  the  dorsal  root  fibers  and  from  the  longitudinal 
fibers  of  the  cord,  as  well  as  terminal  branches  of  these  fibers,  enter  the  gray 
substance  and  ramify  extensively  within  it,  entering  into  synaptic  relations 
with  the  neurons  which  it  contains.  The  branches  of  the  myelinated  fibers 
soon  lose  their  sheaths,  and  it  is  this  relative  scarcity  of  myelin  which  gives  to 


88  THE    NERVOUS    SYSTEM 

this  substance  its  gray  appearance.  The  ramification  of  dendrites  and  unmy- 
elinated fibers  forms  a  very  intricate  feltwork  throughout  the  gray  substance 
(Fig.  64). 

The  nerve-cells  of  the  spinal  cord  vary  greatly  in  size.  The  largest  are 
situated  in  the  anterior  column  and  may  measure  more  than  100  micra.  They 
are  all  multipolar,  possess  each  a  single  axon,  and  may  be  classified  in  four  groups: 
(1)  Some  of  the  cells,  found  in  the  posterior  horn  and  particularly  in  the  sub- 
stantia gelatinosa  Rolandi,  belong  to  Golgi's  Type  II.  with  short  axons  confined 
to  the  gray  substance.  These,  however,  are  present  in  relatively  small  numbers 
in  the  spinal  cord.     (2)  The  motor  cells,  situated  in  the  anterior  column  and 


>;■' 


Fig.  64.— From  a  section  through  the  spinal  cord  of  a  monkey;  showing  part  of  the  an- 
terior gray  column  including  a  multipolar  nerve-cell  and  the  surrounding  neuropil.  Pyridin- 
silver  method. 

most  numerous  in  the  cervical  and  lumbar  enlargements,  are  of  large  size  and 
possess  axons  which  leave  the  cord  in  the  ventral  roots.  (3)  Smaller  cells  are 
present  in  the  lateral  column  in  the  thoracic  region  and  give  rise  to  the  visceral 
efferent  fibers  of  the  ventral  roots  (Fig.  37).  (4)  Other  cells  of  small  or  medium 
size,  found  chiefly  in  the  posterior  column,  possess  axons  which  pass  into  the 
white  matter,  where  they  bend  sharply  to  become  ascending  or  descending 
fibers,  or  divide  dichotomously  into  ascending  and  descending  branches  (Fig. 
68).  Some  of  the  ascending  fibers  reach  the  brain;  the  others  merely  connect 
the  different  levels  of  the  spinal  cord.  The  fibers  of  the  latter  group  constitute 
the  fasciculi  proprii  and   vary  greatly  in  length,   some  connecting  adjacent, 


mi:    SPINAL   CORD 

others,  more  remote,  segments.  Their  collateral  and  terminal  branch) 
enter  and  ramify  within  the  gray  substance.  Those  which  remain  throughout 
in  the  same  lateral  half  of  the  cord  are  called  association  fibers;  while  others, 
known  as  commissural  fibers,  cross  the  median  plane  chiefly  in  the  white  com- 
missure (Fig.  68).  Some  of  the  commissural  fibers  are  -hurt  and  confined  to  a 
single  level  of  the  cord  I  Fig.  66). 

Cell-columns.— The  nerve-cells  are  not  uniformly  distributed  throughout 
the  gray  matter,  for  many  of  them  are  arranged  in  longitudinal  cell-columns. 
In  transverse  sections  each  of  these  columns  appears  as  a  distinct  group  oi 
cells,  somewhat  separated  from  other  similar  groups  within  the  gray  matter 
Fig.  65).  The  large  motor  cells  of  the  anterior  column,  which  give  origin  to 
the  ventral  root  fibers,  form  several  subgroups.  One  of  these,  known  as  the 
anteromedian  cell-column,  occupies  the  medial  part  of  the  anterior  column  through- 
out almost  its  entire  length,  being  absent  only  in  the  fifth  lumbar  and  first 
sacral  segments.  Behind  it  is  the  posteromedian  cell-column,  which  is.  however, 
present  only  in  the  thoracic  and  first  lumbar  segments  and  for  a  short  -tretch 
in  the  cervical  region.  The  axons  from  these  two  medial  groups  of  cells  prob- 
ably supply  the  musculature  of  the  trunk.  In  the  cervical  and  lumbar  enlarge- 
ments there  are  laterally  placed  groups  of  cells  the  axons  of  which  supply  the 
muscles  of  the  limbs.  These  are:  (1)  the  anterolateral  cell-column,  present  in 
the  fourth  to  the  eighth  cervical  and  in  the  second  lumbar  to  the  second 
sacral  segments;  (2)  the  posterolateral  cell-column  in  the  last  live  cervical, 
last  four  lumbar,  and  first  three  sacral  segments;  (3)  the  retro  posterolateral 
cell-column  in  the  eighth  cervical,  first  thoracic,  and  first  three  sacral  seg- 
ments, and  (-T)  the  central  cell-column  in  the  second  lumbar  to  the  second  sacral 
segments. 

The  intcrmcdiolatcral  cell-column  is  found  in  the  lateral  column  in  the  tho- 
racic region  of  the  cord  and  is  prolonged  downward  into  the  upper  lumbar  seg- 
ments. It  is  composed  of  small  cells,  the  axons  of  which  run  through  the  ven- 
tral roots,  spinal  nerves,  and  white  rami  communicantes  into  the  sympathetic 
nervous  system  (Fig.  37).  They  have  to  do  with  the  innervation  of  smooth 
and  cardiac  muscle  and  glandular  tissue.  The  longitudinal  extent  of  this 
column  corresponds  quite  accurately  to  that  of  the  spinal  origin  of  the  white 
rami.  A  group  of  cells,  having  a  similar  function,  is  also  found  in  the  third 
and  fourth  sacral  segments. 

The  cells  of  the  posterior  gray  column  are  smaller,  as  a  rule,  than  those  of  the 
ventral  column;  and  except  for  the  nucleus  dorsalis  they  are  not  arranged  in 


9° 


THE    NERVOUS    SYSTEM 


definite  groups.     They  are  concerned  with  the  reception  and  distribution  of 
the  impulses  entering  along  the  libers  of  the  dorsal  roots. 


S2/W 


S4- 


Fig.  65. — Outline  sketches  of  ventral  horn  of  left  side  of  cord  at  different  levels,  showing  the 
relative  number  and  position  of  the  chief  cell-groups:  G,  G,  Tk,  etc.,  indicate  the  segments — e.  g., 
first  cervical,  fourth  cervical,  sixth  thoracic;  G  (b),  lower  part  of  eighth  cervical.  The  following 
letters  designate  the  cell-groups:  v-m,  Anteromedian;  d-m,  posteromedian;  v-l,  anterolateral; 
d-l,  posterolateral;  p.  d-l,  retroposterolateral;  v  in  L^,  Lit  ventral;  c  in  L2,  La,  Si,  central;  /.  c.  in 
Tf,,  Tn,  intermediolateral;  ace.  in  G,  C4,  accessorius;  phr.  in  C\,  phrenic;  Cl.c.  in  T6,  Tn,  nucleus 
dorsalis.     (Bruce,  Quain's  Anatomy.) 

The  nucleus  dorsalis,  or  column  of  Clarke,  is  a  group  of  large  cells  in  the 
medial  part  of  the  base  of  the  posterior  column.     It  extends  from  the  last  cer- 


THE    SPINAL   (  I  IRD 


91 


vical  or  first  thoracic  to  the  second  or  third  lumbar  segments.  It  is  a  prom 
inent  feature  in  cross-sections  of  the  thoracic  cord,  appearing  as  a  well  denned 
oval  area  richly  supplied  with  collaterals  from  the  dorsal  root..  The  cells  have 
an  oval  or  pyriform  shape;  each  has  several  dendritic  processes  and  an  axon 
which  enters  the  lateral  funiculus,  within  which  it  runs  toward  the  cerebellum 
in  the  dorsal  spinocerebellar  tract. 

The  Spinal  Reflex  Mechanism. — In  the  next  chapter  we  will  consider  at 
length  the  long  ascending  and  descending  paths  in  the  white  substance  of  the 
cord  by  which  afferent  impulses  from  the  spinal  nerves  reach  the  brain,  and 
those  through  which  the  motor  centers  of  the  brain  exert  in  return  a  controlling 
influence  over  the  spinal  motor  apparatus.  But  fully  as  important  as  these  are 
the  purely  intraspinal  connections — the  spinal  reflex  mechanism. 


Fig.  66. — Diagrammatic  section  through  the  spinal  cord  and  a  spinal  nerve  to  illustrate  a 
simple  reflex  arc:  a,  b,  c,  and  d,  Branches  of  sensory  fibers  of  the  dorsal  roots;  e,  association  neuron; 
/,  commissural  neuron. 

A  reflex  arc  in  its  simplest  form  may  be  made  up  of  only  two  neurons,  the 
primary  sensory  and  motor  neurons  wdth  a  synapse  in  the  gray  matter  of  the 
anterior  column  (Fig.  66).  It  consists  of  the  following  parts:  (1)  a  receptor, 
the  peripheral  sensory  endings;  (2)  a  conductor,  the  afferent  nerve-fiber;  (3)  a 
center,  including  the  synapse  in  the  anterior  column;  (4)  a  second  conductor, 
the  efferent  nerve-fiber,  and  (5)  an  effector,  the  muscle-fiber.  Usually,  how- 
ever, there  are  interposed  between  the  primary  sensory  and  motor  elements 
one  or  more  intermediate  neurons.  These,  when  restricted  to  one  side  of  the 
cord,  are  known  as  association  neurons;  when  their  axons  cross  the  median 
plane,  as  many  of  them  do  through  the  anterior  white  commissure,  they  are 
called  commissural  neurons.     When  the  circuit  is  complete  within  a  single  neural 


Q2 


TIIK    XKRVOUS    SYSTEM 


segment  it  may  be  said  to  be  intrasegmental  (Fig.  66);  if  it  extends  through  two 
or  more  such  segments  it  is  an  intersegmental  reflex  arc. 

Intersegmental  Reflex  Arcs. — Impulses  entering  the  spinal  cord  through  a 
given  dorsal  root  may  be  transmitted  to  the  primary  motor  neurons  of  another 
segment  in  one  of  two  ways:  (1)  by  way  of  the  ascending  and  descending  branches 
of  the  dorsal  root  fibers,  and  (2)  along  the  fibers  of  the  fasciculi  proprii  (Fig.  67). 
A  full  account  of  these  two  pathways  will  be  presented  in  the  next  chapter, 
but  a  word  of  explanation  is  required  here.     The  fibers  of  the  dorsal  root  divide, 


Fig.  67. — Diagram  of  the  spinal  cord,  showing  the  elements  concerned  in  a  diffuse  unilat- 
eral reflex:  a,  Spinal  ganglion  cell;  b,  motor  cell  in  anterior  column;  c,  association  neuron. 
(Cajal.) 

soon  after  their  entrance  into  the  cord,  into  long  ascending  and  shorter  descend- 
ing branches,  which  together  form  the  greater  part  of  the  posterior  funiculus 
and  give  off  many  collaterals  to  the  gray  matter  of  the  successive  levels  of  the 
cord  (Fig.  67).  Many  of  the  ascending  branches  reach  the  brain;  but  the  others 
terminate,  as  do  the  descending  branches  and  all  the  collaterals,  in  the  gray 
matter  of  the  cord  (Fig.  68).  The  fasciculi  proprii  immediately  surround  the 
gray  columns  (Fig.  68)  and  consist  of  ascending  and  descending  fibers,  which 
arise  and  terminate  within  the  gray  substance  of  the  cord.  Most  of  these 
fibers  remain  on  the  same  side  as  association  fibers  concerned  in  unilateral  re- 


THE    SPINAL   «  i  'li. 


flexes.  Others  cross  in  the  anterior  white  commissure  and  are  commi  ural 
libers  concerned  in  crossed  reflexes.  Afferent  impulses  ma)  be  transmitted 
along  tin'  cord  in  either  direction  b)  the  branches  oJ  the  dorsal  root  fibers;  or  by 
means  of  synapses  in  the  gray  matter  they  may  be  transferred  to  the  long 
eiation  and  commissural  fibers  and  conveyed  to  the  primary  motor  neurons  oi 
the  same  or  opposite  side  in  more  or  less  distant  segments.  The  course  of  a 
nerve  impulse  in  a  unilateral  intersegmental  reflex  is  indicated  on  the  Left  side 


Dorsal  root 


Ventral  root 


Ascending  branch  of  dorsal  root  fiber 

.  I  ssociation  fibers  -'-'"_ 
Descending  branch  of  dorsal  foot  fiber 


Dorsal  root 


-■Commissural  fibers 


Ventral  root 


Fig.  68.— Diagram  of  the  spinal  cord,  showing  the  elements  concerned  in  intersegmental  reflexes. 

of  Fig.  68,  while  on  the  right  side  of  the  same  figure  are  shown  the  elements 
concerned  in  crossed  reflexes. 

The  observations  of  Coghill  (1913  and  1914)  and  of  Herrick  and  Coghill  (1915)  tend  to 
show  that  the  simple  form  of  reflex  arc  illustrated  in  Fig.  66  is  not  the  primitive  type.  In 
larval  Amblystoma  the  first  arcs  to  become  functionally  mature  are  composed  ot  chains 
of  many  neurons,  so  arranged  that  every  cutaneous  stimulus  elicits  the  same  complex  response 
of  the  entire  somatic  musculature,  i.  e.,  the  swimming  movement.  It  is  of  particular  interest 
to  note  that  in  this  primitive  reflex  mechanism  the  sensory  fibers  arise  from  giant  cells  located 
within  the  spinal  cord  and  that  the  ventral  root  fibers  are  collaterals  from  the  central  motor 
tract.  In  adult  Amblystoma  these  sensory  and  motor  elements  are  replaced  by  the  usual 
type  of  primary  sensory  and  motor  neurons. 


94  THE   NERVOUS    SYSTEM 

We  may  mention  as  an  example  of  a  reflex  arc  involving  many  segments  of 
the  cord  the  "scratch-reflex"  of  the  dog,  which  has  been  very  carefully  investi- 
gated by  Sherrington  (1906).  If,  some  time  after  transection  of  the  spinal  cord 
in  the  low  cervical  region,  the  skin  covering  the  dorsal  aspect  of  the  thorax  be 
stimulated  by  pulling  lightly  on  a  hair,  the  hind  limb  of  the  corresponding  side 
begins  a  series  of  rhythmic  scratching  movements.  By  degeneration  experi- 
ments it  was  shown  that  this  reflex  arc  probably  includes  the  following  elements: 
(1)  a  primary  sensory  neuron  from  the  skin  to  the  spinal  gray  matter  of  the 
corresponding  neural  segment;  (2)  a  long  descending  association  neuron  from  the 


Fig.  69. — Diagram  of  the  spinal  arcs  involved  in  the  scratch-reflex:  Ra  and  R3,  Receptive 
paths  from  hairs  in  the  dorsal  skin  of  left  side;  Pa  and  Pft,  association  neurons;  FC,  motor  fibers  of 
ventral  root.      (Sherrington.) 

shoulder  to  the  leg  segments,  and  (3)  a  primary  motor  neuron  to  a  flexor  muscle 
of  the  leg  (Fig.  69). 

A  primary  motor  neuron  seldom,  if  ever,  belongs  exclusively  to  one  arc,  but 
serves  as  the  final  channel  to  which  many  streams  converge.  Its  perikaryon 
gives  off  wide-spread  dendritic  processes,  through  which  it  comes  into  relation 
with  the  ramifications  of  axons  from  many  different  sources.  In  this  way 
impulses  reach  it  from  the  dorsal  roots,  and  from  the  fasciculi  proprii  of  the 
spinal  cord,  as  well  as  from  a  number  of  tracts  which  descend  into  the  spinal 
cord  from  centers  in  the  brain  (the  corticospinal,  rubrospinal,  tectospinal,  and 
vestibulospinal  tracts).  The  primary  motor  neuron  is,  as  Sherrington  has  said, 
"the  final  common  path." 


CHAPTER  VII 

FIBER  TRACTS  OF  THE  SPINAL  CORD 

The  fibers  composing  the  white  substance  of  the  spina]  cord  are  not  scat- 
tered and  intermingled  at  random,  but,  on  the  contrary,  those  of  a  given  func- 
tion are  grouped  together  in  more  or  less  definite  bundles.  A  bundle  of  fibers 
all  of  which  have  the  same  origin,  termination,  and  function  is  known  as  a  fiber 
tract .  The  funiculi  of  the  spinal  cord  are  composed  of  many  such  tracts  of 
longitudinal  fibers,  which,  while  occupying  fairly  definite  areas,  blend  more  or 
less  with  each  other,  in  the  sense  that  there  is  considerable  intermingling  of  the 
fibers  of  adjacent  tracts.  It  is  convenient  to  have  a  name  for  certain  obvious 
subdivisions  of  the  funiculi  which  contain  fibers  belonging  to  more  than  one  tract. 
Such  a  mixed  bundle  is  properly  called  a  fasciculus. 

THE  INTRAMEDULLARY  COURSE  OF  THE  DORSAL  ROOT  FIBERS 

The  central  end  of  a  dorsal  root  breaks  up  into  many  rootlets  or  filaments 
(fila  radicularia),  which  enter  the  spinal  cord  in  linear  order  along  the  line  of 
the  posterior  lateral  sulcus.  As  it  enters  the  cord  each  filament  can  be  seen  to 
separate  into  a  larger  medial  and  a  much  smaller  lateral  division.  The  fibers  of 
the  medial  division  are  of  relatively  large  caliber  and  run  over  the  tip  of  the 
posterior  column  into  the  posterior  funiculus  (Fig.  72).  Those  of  the  lateral 
division  are  fine  and  enter  a  small  fascicle  which  lies  along  the  apex  of  the  pos- 
terior column,  the  fasciculus  dorsolateralis  or  tract  of  Lissauer.  Very  soon 
after  their  entrance  into  the  cord  each  dorsal  root  fiber  divides  in  the  manner  of 
a  Y  into  a  longer  ascending  and  a  shorter  descending  branch  (Fig.  70). 

The  ascending  branches  of  the  fibers  of  the  medial  division  of  the  dorsal  root 
run  for  considerable  but  varying  distances  in  the  posterior  funiculus;  some  from 
each  root  reach  the  medulla  oblongata,  others  terminate  at  different  levels  in  the 
gray  matter  of  the  spinal  cord.  At  the  level  of  their  entry  into  the  cord  these 
fibers  occupy  the  lateral  portion  of  the  fasiculus  cuneatus;  but  in  their  course 
cephalad,  as  each  successive  root  adds  its  quota,  those  from  the  more  caudal 
roots  are  displaced  medianward.  In  this  way  the  longer  fibers  come  to  occupy 
the  medial  portion  of  the  posterior  funiculus  (Fig.  71).     In  the  cervical  region 

95 


96 


THE    NERVOUS    SYSTEM 


the  long  ascending  fibers  from  the  sacral,  lumbar,  and  lower  thoracic  roots 
constitute  a  well-defined  medially  placed  bundle,  the  fasciculus  gracilis,  sepa- 
rated from  the  rest  of  the  posterior  funiculus  by  the  posterior  intermediate 
septum.  Those  of  the  long  ascending  fibers,  which  finally  reach  the  brain, 
terminate  in  gray  masses  in  the  posterior  funiculi  of  the  medulla  oblongata 


Fig.  70. — Bifurcation  of  the  dorsal  root  fibers  within  the  spinal  cord  into  ascending  and 
descending  branches,  which  in  turn  give  off  collaterals;  the  termination  of  some  of  these  col- 
laterals in  synaptic  relation  to  cells  of  the  posterior  gray  column.      (Cajal,  Edinger.) 


(nucleus  of  the  funiculus  gracilis  and  nucleus  of  the  funiculus  cuneatus).  Since 
the  number  of  these  long  ascending  branches  must  increase  from  below  upward 
it  is  easy  to  understand  the  progressive  increase  in  size  of  the  posterior  funiculus 
from  the  sacral  to  the  cervical  region  (Fig.  60). 

The  fasciculus  gracilis  and  fasciculus  cuneatus  are  composed  for  the  most 


i'llil  K     I  R  \i   fS    01    NIK    SPINAL   CORD 


97 


/     i .  gracilis 
I  a       i  Utieatui 


part  of  these  ascending  branches  of  the  dorsal  root  fibers,  the  former  contain 
Ing  those  which  have  the  Longest  intramedullary  course. 

The  descending  branches  of  the  fibers  of  the  medial  division  of  the  dorsal 
mot  are  all  relatively  short.  The  shortest  terminate  at  once  in  the  gray  matter 
dt"  the  posterior  column.  Others  descend  in  the  fasciculus  inter/ ascicularis,  or 
comma  tract  of  Schultze,  which  is  situated  near  the  center  of  the  posterior  fu- 
niculus; and  still  others  run  near  the  posterior  median  septum  in  the  septomar- 
ginal fasciculus  (Fig.  76).  In  both  of  these  fas- 
cicles they  arc  intermingled  with  descending  fillers, 
arising  from  cells  within  the  gray  matter  of  the  spinal 
cord. 

Collaterals. — At  intervals  along  both  ascending 
and  descending  branches  collaterals  are  given  off  which 
run  ventrally  to  end  in  the  gray  matter  (Fig.  70). 
They  are  much  liner  than  the  fibers  from  which  they 
arise,  and  the  total  number  arising  from  a  given  fiber 
is  rather  large.  Some  of  them  end  in  the  ventral 
gray  column;  others,  in  the  posterior  gray  column, 
including  the  substantia  gelatinosa  and  the  nucleus 
dorsalis;  still  others  run  through  the  dorsal  com- 
missure to  the  opposite  side  of  the  cord,  where  they 
appear  to  end  in  the  posterior  columns  (Fig.  72).  In 
Fig.  70  there  are  illustrated  the  arborizations  formed 
by  some  of  these  collaterals  about  cells  of  the  posterior 
column. 

The  terminals  of  the  descending  branches  and  of 
those  ascending  branches,  which  do  not  reach  the  brain, 
end  as  do  the  collaterals  within  the  gray  matter  of  the 
spinal  cord. 

The  fibers  of  the  lateral  division  of  the  dorsal  root  are  all  very  fine.  The 
majority  are  unmyelinated  and  can  be  recognized  only  in  preparations  in  which 
the  axons  are  stained.  A  good  account  of  their  appearance  in  Golgi  prepara- 
tions has  been  given  by  Barker  (1899,  pp.  466-468).  In  Weigert  preparations 
we  must  look  carefully  to  find  the  few  myelinated  fibers  contained  in  this  divi- 
sion. But  in  pyridin-silver  preparations  great  numbers  of  fine  unmyelinated 
fibers,  accompanied  by  a  few  which  are  myelinated,  can  be  seen  to  turn  lateral- 
ward  as  the  root  filament  enters  the  cord.     These  constitute  the  lateral  division 

7 


Fig.  71. — Diagram  to 
illustrate  the  arrangement 
of  the  ascending  branches 
of  the  dorsal  root  fibers 
within  the  posterior  funic- 
ulus of  the  spinal  cord. 


98 


THE    NERVOUS    SYSTEM 


of  the  root  and  enter  the  dorsolateral  fasciculus  or  tract  of  Lissauer  (Fig.  72). 
The  medial  division,  on  the  other  hand,  consists  exclusively  or  almost  exclu- 
sively of  myelinated  fibers.  The  fibers  of  the  lateral  division  of  the  root  divide 
into  ascending  and  descending  branches,  both  of  which,  however,  are  very 
short.  The  ascending  branch,  which  is  the  longer  of  the  two,  does  not  extend 
at  most  more  than  the  length  of  one  or  two  segments  in  the  long  axis  of  the 
cord  (Ranson,  1913,  1914). 

The  dorsolateral  fasciculus,  or  tract  of  Lissauer,  lies  between  the  apex  of 
the  posterior  column  and  the  periphery  of  the  cord,  and  varies  greatly  in  shape 
and  size  in  the  different  levels  of  the  cord  (Figs.  55-58).     It  is  composed  of 


Medial  division  of  dorsal  root 


his  cuncatus 

solalcral 
isciculus 

Dorsal  spino- 
cerebellar tract 


Dorsal  spinocc 

\i  {       Vf^^ ^-^       >J 

Ventral  spino- 
cerebellar tract 
Lateral  spino- 
thalamic and 
spinotectal  tracts 
Ventral  spinothalamic 

Fig.  72. — Diagram  of  the  spinal  cord  and  dorsal  root,  showing  the  divisions  of  the  dorsal  root, 
the  collaterals  of  the  dorsal  root  fibers,  and  some  of  the  connections  which  are  established  by 
them. 

unmyelinated  and  fine  myelinated  fibers,  which  are  derived  in  part  from  the 
lateral  division  of  the  dorsal  root  and  in  part  arise  from  cells  in  the  neighboring 
gray  matter  (Fig.  63). 


AFFERENT  PATHS  IN  THE  SPINAL  CORD 

We  have  been  at  some  pains  to  make  clear  the  course  and  distribution  of 
the  dorsal  root  fibers  within  the  spinal  cord  because  all  afferent  impulses  which 
reach  the  cord  are  carried  by  them.  Interoceptive  fibers  from  the  viscera, 
proprioceptive  fibers  from  the  muscles,  tendons,  and  joints,  as  well  as  extero- 
ceptive fibers  from  the  skin  are  included  in  these  roots;  and  among  the  latter 
group  are  probably  several  subvarieties,  mediating  the  afferent  impulses  out 


ETBEK    TRACTS    OF    ill::   SPINAL   CORD 

of  which  the  sensations  of  touch,  heatj  cold,  and  pain  arc  elaborated.  An 
important  problem  which  in  great  measure  awaits  solution  is  this:  How  are  the 
-fibers  of  the  different  functional  varieties  distributed  in  the  spinal  cord  and 
along  what  paths  are  these  various  types  of  afferent  impulses  carried  toward 
the  brain? 

The  proprioceptive  fibers,  which  terminate  at  the  periphery  in  neuromus 
cular  and  neurotendinous  spindles  and  in  Pacinian  corpuscles,  arc  known  to 
be  myelinated.  They  must,  therefore,  pass  through  the  well  myelinated  medial 
division  of  the  dorsal  root  into  the  posterior  funiculus.  As  shown  by  Brown 
Sequard  in  1847  by  a  stud}-  of  patients  with  unilateral  lesions  of  the  spinal 
cord,  sensations  from  the  muscles,  joints,  and  tendons  reach  the  brain  without 
undergoing  a  crossing  in  the  spinal  cord.  This  and  other  evidence  points  un- 
mistakably to  the  long  ascending  branches  of  the  dorsal  root  fibers,  which  are 
continued  uncrossed  in  the  posterior  funiculus  to  the  medulla  oblongata,  as  the 
conductors  of  this  type  of  sensation.  When  these  fibers  are  destroyed  by  a 
tumor  or  other  lesion  confined  to  the  posterior  funiculus,  muscular  sensibility 
and  the  recognition  of  posture  are  abolished,  while  touch,  pain,  and  tempera- 
ture sensations  remain  intact  (Dejerine,  1914). 

No  better  exposition  of  the  proprioceptive  functions  could  be  furnished  than 
by  describing  the  sensory  deficiencies  found  in  cases  of  tabes  dorsalis  or  loco- 
motor ataxia,  a  disease  in  which  there  is  degeneration  of  the  posterior  funiculi. 
Lying  in  bed,  with  eyes  closed,  a  tabetic  may  not  be  able  to  say  in  wrhat  posi- 
tion his  foot  has  been  placed  by  an  attendant  because  afferent  impulses  from 
the  muscles,  joints,  and  tendons  fail  to  reach  the  cerebral  cortex  and  arouse 
sensations  of  posture.  Not  only  are  the  sensations  of  this  variety  lacking,  but 
the  unconscious  reflex  motor  adjustments  initiated  by  proprioceptive  afferent 
impulses  are  also  impaired.  Standing  with  feet  together  and  eyes  closed,  the 
patient  loses  his  balance  and  sways  from  side  to  side.  In  walking  his  gait  is 
uncertain  and  the  movements  of  his  limbs  poorly  coordinated.  All  of  this 
motor  incoordination  is  explained  by  a  loss  of  the  controlling  afferent  impulses 
from  the  muscles,  joints,  and  tendons. 

The  long  ascending  fibers  of  the  posterior  funiculus,  which  reach  the  brain 
and  end  in  the  nucleus  gracilis  and  cuneatus,  are  for  the  most  part  proprio- 
ceptive in  function  (Fig.  235).  The  connections  which  they  make  there  can 
best  be  considered  in  another  chapter.  Collaterals  and  many  terminal  branches 
end  in  the  gray  matter  of  the  cord,  entering  into  synaptic  relations  with  the  neu- 
rons of  the  spinocerebellar  paths  and  with  neurons  belonging  to  spinal  reflex  arcs. 


IOO  THE   NERVOUS    SYSTEM 

Proprioceptive  Paths  to  the  Cerebellum. — According  to  the  researches  of 
Marburg  (1904)  and  Bing  (1906)  the  spinocerebellar  tracts  are  concerned  with 
the  transmission  to  the  cerebellum  of  afferent  impulses  from  the  muscles,  joints, 
and  tendons,  which  remain,  however,  at  a  subconscious  level  (Dejerine,  1914). 
We  may,  therefore,  appropriately  consider  these  paths  at  this  time. 

The  dorsal  spinocerebellar  tract  (fasciculus  spinocerebellaris  dorsalis,  direct 
cerebellar  tract  of  Flechsig,  fasciculus  cerebellospinalis)  is  a  well-defined  bundle 
at  the  surface  of  the  lateral  funiculus  just  ventral  to  the  posterior  lateral  sul- 
cus (Figs.  72,  78).  In  cross-section  it  has  the  form  of  a  flattened  band,  situated 
between  the  periphery  of  the  cord  and  the  lateral  corticospinal  tract.  It  begins 
in  the  upper  lumbar  segments  and  is  prominent  in  the  thoracic  and  cervical 
portions  of  the  cord.  It  consists  of  uniformly  large  fibers,  which  take  origin 
from  the  cells  of  the  nucleus  dorsalis  of  the  same  side.  This  nucleus  forms  a 
prominent  feature  of  the  sections  through  the  thoracic  portion  of  the  cord,  but 
is  not  found  above  the  seventh  cervical  nor  below  the  second  lumbar  seg- 
ments. A  conspicuous  bundle  of  myelinated  collaterals  from  fibers  of  the 
fasciculus  cuneatus  run  to  this  nucleus  (Fig.  56)  where  their  arborizations  form 
baskets  about  the  individual  cells  of  the  nucleus.  The  fibers  arising  from  the 
cells  of  the  nucleus  dorsalis  run  laterally  to  the  periphery  of  the  lateral  funiculus 
of  the  same  side,  where  they  turn  rostrally  and  form  the  dorsal  spinocerebellar  tract. 
We  will  follow  this  tract  into  the  brain  in  a  later  chapter.  Here  we  need  only 
say  that  it  reaches  the  cerebellum  by  way  of  the  restiform  body  (Fig.  235). 

The  ventral  spinocerebellar  tract  constitutes  the  more  superficial  portion  of 
a  large  ascending  bundle  of  fibers,  known  as  the  fasciculus  anterolateralis  super- 
ficialis  or  Gower's  tract,  which  also  includes  the  spinotectal  and  lateral  spino- 
thalmic  tracts  (Fig.  72).  It  is  situated  at  the  periphery  of  the  lateral  funiculus 
ventral  to  the  tract  we  have  just  considered.  It  is  said  to  consist  of  fibers  which 
arise  from  the  cells  of  the  posterior  gray  column  and  intermediate  gray  matter  of  the 
same  and  the  opposite  side  (Page  May,  1906;  Dejerine,  1914).  In  a  subsequent 
chapter  we  will  trace  these  fibers  by  the  way  of  the  medulla,  pons,  and  an- 
terior medullary  velum  to  the  cerebellum  (Fig.  235). 

From  what  has  been  presented  above  it  will  be  apparent  that  collaterals 
and  terminal  branches  of  dorsal  root  fibers,  doubtless  of  the  proprioceptive 
group,  enter  into  synaptic  relations  with  certain  intraspinal  neurons,  the  axons 
of  which  run  to  the  cerebellum  by  way  of  the  ventral  and  dorsal  spinocerebellar 
tracts.  The  entire  path  from  periphery  to  cerebellum  therefore  consists  of  two 
neurons  with  a  synaptic  interruption  in  the  gray  matter. 


I  ir.i  K     l  K At  is    OP    i  in;    SPIN  \i.   CORD  ioi 

Interoceptive  fibers  arc  present  in  the  thoracic  and  upper,  lumbar  doi  I 
roots,  hut  are  either  absent  or  verj  few  in  number  in  the  others.  We  know 
practically  nothing  about  their  intraspinal  course  in  mammals.  The}  will  be 
considered  In  the  chapter  on  the  Sympathetic  Nervous  System. 

Exteroceptive  fibers  carry  cutaneous  afferent  impulses,  and  probably  are 
subdivided  into  several  varieties.  Most  authors  agree  that  there  are  separate 
fibers  for  the  impulses  aroused  by  tactile  and  thermal  stimuli;  and  Sherrington 
1906)  lias  presented  evidence  for  the  existence  of  a  separate  group  of  Gibers, 
whose  end  organs  are  responsive  only  to  agents  capable  of  inflicting  injury, 
that  is,  to  noxious  or  painful  stimuli. 

Conduction  of  Tactile  Impulses  in  the  Spinal  Cord. — The  phenomena  of  sen- 
sory dissociation,  characteristic  of  syringomyelia,  show  that  the  intraspinal 
path  for  the  sensations  of  touch  is  rather  widely  separated  from  that  for  pain 
and  temperature  sensation  (Fig.  73).  In  that  disease  a  cavity  is  developed 
within  the  gray  matter  of  the  spinal  cord;  and  sensations  of  pain  and  tem- 
perature may  be  abolished  over  a  given  cutaneous  area  which  is  still  sensitive 
to  touch.  The  separation  of  these  two  lines  of  conduction  occurs  at  the  place 
where  the  dorsal  root  fibers  enter  the  cord.  The  fibers,  mediating  pain  and 
temperature  sensations,  end  almost  at  once  in  the  gray  matter,  while  those  for 
touch  ascend  for  some  distance  in  the  posterior  funiculus  of  the  same  side  (Head 
and  Thompson,  1906;  Dejerine,  1914).  As  these  fibers  ascend  in  the  posterior 
funiculus  they  give  off  collaterals  to  the  gray  matter  of  the  successive  levels  of 
the  spinal  cord  through  which  they  pass.  The  tactile  impulses  from  a  given 
root,  therefore,  do  not  enter  the  gray  matter  all  at  once,  but  filter  forward  through 
the  collaterals  and  terminals  of  these  dorsal  root  fibers  to  reach  the  posterior 
gray  column  in  a  considerable  number  of  segments  above  that  at  which  the 
root  enters  the  cord.  Within  the  posterior  gray  column  at  these  successive 
levels  the  terminals  and  collaterals  of  the  tactile  fibers  establish  synaptic  con- 
nections with  neurons  of  the  second  order.  The  axons  of  these  neurons  form  the 
ventral  spinothalamic  tract  of  the  opposite  side  (Fig.  73). 

The  ventral  spinothalamic  tract  is  an  ascending  bundle  of  fibers  found  in  the 
anterior  funiculus.  It  consists  of  fibers  which  take  origin  from  cells  in  the  pos- 
terior column  of  the  opposite  side,  cross  the  median  plane  in  the  anterior  white 
commissure,  and  ascend  in  the  ventral  funiculus  to  end  within  the  thalamus  (Fig. 
73).  It  is  possible  that  many  of  the  fibers  do  not  reach  the  thalamus  directly, 
but  terminate  in  the  gray  matter  of  the  cord  and  medulla  oblongata  in  rela- 
tion to  other  neurons,  whose  axons  continue  the  course  to  the  thalamus.     If 


102 


THE   NERVOUS    SYSTEM 


this  be  so  the  path  consists  in  part  of  relays  of  shorter  neurons  (Dejerine, 

1914). 

The  uncrossed  path  in  the  posterior  funiculus  for  tactile  impulses  entering 
the  cord  through  any  given  dorsal  root  overlaps  by  many  segments  the  crossed 
path  in  the  ventral  funiculus  (Fig.  230).  Some  of  the  uncrossed  fibers  even 
reach  the  nuclei  of  the  funiculus  gracilis  and  funiculus  cuneatus  in  the  medulla 
oblongata.  This  extensive  overlapping  of  the  uncrossed  by  the  crossed  paths 
accounts  for  the  fact  that  lateral  hemisection  of  the  human  spinal  cord  rarely 
causes  marked  disturbance  of  tactile  sensibility  below  the  lesion  (Petren,  1902; 
Head  and  Thompson,  1906). 

ll 


Ascending  branch  of  dorsal  root  fiber  - 
Myelinated  fiber  of  dorsal  rooty 
Spina!  ganglion 

Unmyelinated  fiber  of  dorsal  root 


Lateral  spinothalamic  tract 
(pain  and  temperature) 

Ventral  spinothalamic  tract 
(touch) 


Fig.  73. — Exteroceptive  pathways  in  the  spinal  cord. 

Since  it  seems  clear  that  the  dorsal  root  fibers  subserving  tactile  sensibility  ascend  for 
some  distance  in  the  posterior  funiculus,  they  must  be  included  among  the  myelinated  fibers 
of  the  medial  division  of  the  dorsal  root,  because  only  myelinated  fibers  ascend  in  that 
funiculus.  This  conclusion  is  in  keeping  with  the  facts  already  mentioned  concerning  the 
termination  of  myelinated  fibers  in  the  supposedly  tactile  end  organs,  such  as  Meissner's 
corpuscles  and  Pacinian  corpuscles.  It  is  also  in  keeping  with  facts  to  be  mentioned  in 
a  following  paragraph  concerning  the  structure  of  the  median  nerve. 

The  Lateral  Spinothalamic  Tract. — It  seems  to  be  well  established  that  the 
dorsal  root  fibers,  which  serve  as  pain  conductors,  terminate  in  the  gray  matter 
almost  at  once  after  entering  the  cord,  and  come  into  synaptic  relations  with 
neurons  of  the  second  order,  whose  axons  run  in  the  lateral  spinothalamic  tract. 
From  cells  in  the  posterior  column  fibers  arise,  which  in  man  cross  to  the  opposite 
side  of  the  cord  in  the  anterior  white  commissure  and  ascend  in  the  lateral  spino- 
thalamic tract  to  end  in  the  thalamus  (Figs.  73,  231).    This  is  a  tract  of  ascending 


FIBER    TRACTS    OF    III!.    SPINAL   CORD  103 

fibers  situated  in  the 'lateral  funiculus  under  cover  of  the  ventral  spinocerebellar 
tract.  Together  with  the  spinotectal  and  ventral  spinocerebellar  tracts  it  forms 
the  fasciculus  anterolateral  superncialis  (of  Gowers).     It  mediat*     pain  and 

temperature  sensations. 

Conduction  of  Painful  Afferent  Impulses  iii  the  Spinal  Cod.  Nol  all  of  the  fibers  of 
the  lateral  spinothalamic  Iran  reach  the  thalamus.  According  to  May  (1906),  "Some  of 
these  fibers  certainly  pass  directly  to  the  thalamus,  while  others  terminate  in  the  inter- 
mediate gray  matter,  and  thus,  by  means  of  a  series  of  short  chains,  afford  secondary  paths 
to  the  same  end  station,  which  may  supplement  the  direct  path,  or  be  made  available  after 
interruption  of  the  direct  path."  It  has  been  shown  in  many  cases  in  man  and  animals  that, 
after  a  complete  hemisection  of  the  spinal  cord,  the  loss  of  sensibility  to  pain  on  the  op- 
posite side  of  the  body  below  the  lesion  was  only  temporary.  In  time  there  may  occur  a 
more  or  less  perfect  restoration  of  pain  conduction,  showing  that  the  homolateral  side  of 
the  cord  is  able  to  supplement  or  replace  the  heterolateral  path.  According  to  the  researches 
of  Karplus  and  Kreidl  (1914)  and  Ranson  and  Billingsley  (1916)  these  short  chains,  which  are 
of  secondary  importance  in  man,  are  much  better  developed  in  the  cat.  In  this  animal 
pain  conduction  through  the  spinal  cord  is  bilateral  and  is  effected  to  a  large  extent  through 
a  series  of  short  relays. 

According  to  Head  and  Thompson  (1906)  the  path  for  pain  in  the  spinal  cord  is  the  same 
whether  the  impulses  arise  in  the  skin  or  in  the  deeper  parts,  such  as  the  muscles  and  joints. 
But  Dejerine  (1914)  is  of  the  opinion  that  painful  impulses  from  the  muscles  may  be  trans- 
mitted in  the  posterior  funiculus  and  remain  uncrossed  as  far  as  the  medulla  oblongata. 

Until  recently  we  possessed  no  information  as  to  which  dorsal  root  fibers  served  as  pain 
conductors.  But  in  the  last  few  years  evidence  has  been  presented  which  points  toward  the 
unmyelinated  fibers  of  the  spinal  nerves  and  dorsal  roots  as  the  pain  fibers  (Ranson,  1915). 
Space  does  not  permit  a  detailed  presentation  of  the  evidence  here.  It  should  be  noted, 
however,  that  the  unmyelinated  fibers  of  the  lateral  division  of  the  dorsal  root  terminate  in 
the  gray  matter  almost  immediately  after  their  entrance  into  the  spinal  cord,  and  in  this 
respect  correspond  to  the  known  course  of  the  fibers  carrying  painful  impulses.  The  un- 
myelinated fibers  are  chiefly  distributed  in  the  cutaneous  nerves,  although  a  few  run  in  the 
muscular  branches.  This  coincides  with  the  much  greater  sensitiveness  to  pain  of  the 
skin  than  of  the  deeper  tissues.  Furthermore,  the  median  nerve  at  the  wrist,  a  large  nerve 
supplying  a  relatively  small  area  of  skin  richly  endowed  with  the  sense  of  touch,  contains 
relatively  few  unmyelinated  fibers.  On  the  other  hand,  nerves  like  the  lateral  cutaneous 
of  the  thigh  and  the  medial  cutaneous  of  the  forearm,  which  supply  relatively  large  cutaneous 
areas  of  low  tactile  sensibility,  but  not  inferior  to  the  fingers  in  sensitiveness  to  pain,  are  com- 
posed in  large  part  of  unmyelinated  fibers.  This  difference  between  the  composition  of  the 
median  nerve  and  the  medial  cutaneous  nerve  of  the  forearm  is  just  what  should  be  expected 
if  the  touch  fibers  are  myelinated  and  the  pain  fibers  unmyelinated.  Head  and  his  co-workers 
(1905,  1906.  1908)  have  regarded  the  group  of  sensations  (protopathic),  to  which  according 
to  their  classification  cutaneous  pain  belongs,  as  primitive  in  character  and  the  first  to  appear 
in  the  phylogenetic  series.  It  is  well  known  that  nerve-fibers  in  their  earliest  phylog' 
are  unmyelinated.  If  our  conception  is  correct,  a  great  many  of  the  afferent  fibers  of  mam- 
mals remain  in  this  primitive  undifferentiated  state  and  mediate  a  relatively  primitive 
form  of  sensation.  In  this  connection  it  is  interesting  to  note  that  Dejerine  1914)  believes 
that  pain  is  conducted  by  the  "sympathetic"  fibers  contained  in  the  cutaneous  and  muscular 
nerves.  He  does  not  state  the  evidence  on  which  this  belief  is  based;  but  if  by  "sympathetic" 
he  means  to  designate  the  unmyelinated  fibers  his  view  agrees  perfectly  with  that  presented 
in  the  preceding  paragraphs. 


io4 


THE    NERVOUS    SYSTEM 


The  problem  can  be  approached  from  the  experimental  standpoint.  The  seventh  lum- 
bar dorsal  root  of  the  cat  is  especially  adapted  for  such  a  test.  This  root  as  it  approaches 
the  cord  breaks  up  into  a  number  of  filaments  which  spread  out  in  a  longitudinal  direction 
and  enter  the  cord  along  the  posterolateral  sulcus.  Within  each  root  filament,  as  it  ap- 
proaches this  sulcus,  the  unmyelinated  separate  out  from  among  the  myelinated  fibers  and 
take  up  a  position  around  the  circumference  of  the  filament  and  along  septa  that  divide  it 
into  smaller  bundles.  As  the  root  enters  the  cord,  these  unmyelinated  fibers  turn  laterally 
into  the  dorsolateral  fasciculus,  constituting  together  with  a  few  fine  myelinated  fibers  the 
lateral  division  of  the  root  (Fig.  74).  Almost  all  of  the  myelinated  fibers  run  through  the 
medial  division  of  the  root  into  the  cuneate  fasciculus.     A  slight  cut  in  the  direction  of  the 


Posterior  [utuculus. 


Unmueli rated,  [iters. 
Lissauers  tract 
Dorsal  , 


Fig.  74. — From  a  section  of  the  seventh  lumbar  segment  of  the  spinal  cord  of  the  cat,  showing  the 
unmyelinated  fibers  of  the  dorsal  root  entering  the  tract  of  Lissauer. 


arrow,  which  as  shown  by  subsequent  microscopic  examination  divided  the  lateral  without 
injury  to  the  medial  division  of  the  root,  at  once  eliminated  the  pain  reflexes  obtainable 
from  this  root  in  the  anesthetized  cat,  such  as  struggling,  acceleration  of  respiration,  and 
rise  of  blood-pressure.  On  the  other  hand,  a  long  deep  cut  in  the  plane  indicated  by  B, 
Fig.  74,  which  severed  the  medial  division  of  the  root  as  it  entered  the  cord,  had  little  or  no 
effect  on  the  pain  reflexes.  This  series  of  experiments,  the  details  of  which  are  given  else- 
where (Ranson  and  Billingslcy,  1916),  furnishes  strong  evidence  that  painful  afferent  im- 
pulses are  carried  by  the  unmyelinated  fibers  of  the  lateral  division  of  the  dorsal  root. 

These  fibers  probably  terminate  in  the  substantia  gelatinosa  Rolandi,  and,  if  so,  it  is 
not  unlikely  that  intermediate  neurons  are  intercalated  between  them  and  the  neurons 
whose  axons  run  in  the  ventral  spinothalamic  tra.ct. 


FIBER    TRACTS    0!    THE    SPINAL   CORD 


105 


The  Conduction  of  Sensations  of  Pain,  of  Heat,  and  of  Cold. —  It  is  well  estab- 
lished <m  the  basis  of  clinical  observations  thai  the  paths  for  sensations  of  heat 
ami  cold  follow  closely  those  for  pain.  They  pass  through  the  gra)  matter  im- 
mediately after  entering  the  cord,  cross  to  the  opposite  side,  and  ascend  in  the 
lateral  spinothalamic  trad . 

According  to  May  (1906)  "it  is  clear  thai  there  arc  distincl  and  separate 
paths  for  the  impulses  of  pain,  of  heat,  or  of  cold  in  the  spinal  cord,  and  that 
these  different  and  specific  qualities  of  sensation  may  be  dissociated  in  an  affec- 
tion of  the  spinal  cord."  Thai  is,  one  of  these  forms  of  sensibility  may  be  lost, 
although  the  other  two  are  retained.  "But  as  these  paths  are  anatomically 
very  closely  associated  from  origin  to  termination  these  three  forms  of  sensa- 
tion are  usually  affected  to  a  like  degree." 

From  what  has  been  said  above  it  will  be  apparent  that  the  paths,  mediating 
pain  and  temperature  sensibility,  cross  promptly  to  the  opposite  side  of  the 
cord  and  ascend  in  the  lateral  spinothalamic  tract.  The  path  for  touch  crosses 
more  gradually,  but  finally  comes  to  lie  in  the  ventral  spinothalamic  tract  of 
the  opposite  side;  while  the  sensory  impulses  from  the  muscles,  joints,  and 
tendons,  as  well  as  some  elements  of  tactile  sensibility,  are  carried  upward  on 
the  same  side  of  the  cord  by  the  long  ascending  branches  of  the  dorsal  root  fibers, 
which  terminate  in  the  nuclei  of  the  funiculus  gracilis  and  the  funiculus  cuneatus. 
The  connections  established  within  the  brain  by  the  fibers  of  these  various  paths 
cannot  profitably  be  discussed  at  this  point,  but  will  be  considered  in  Chapter  XIX. 

Other  afferent  paths  besides  those  already  mentioned  exist  in  the  spinal 
cord.  These  include  the  spino-olivary  and  spinotectal  tracts  (Fig.  78).  The 
former  consists  of  fibers  which  arise  from  cells  in  the  posterior  gray  column, 
cross  to  the  opposite  side  of  the  cord,  and  ascend  in  the  ventral  funiculus,  to 
end  in  the  inferior  olivary  nucleus  of  the  medulla  oblongata.  The  spinotectal 
tract  consists  of  fibers  which  arise  from  cells  in  the  posterior  gray  column  and 
which,  after  crossing,  ascend  in  the  late^d  funiculus  in  company  with  those  of 
the  lateral  spinothalamic  path  to  end  in  the  roof  (tectum)  of  the  mesencephalon, 
/'.  e.,  in  the  corpora  quadrigemina. 

ASCENDING  AND  DESCENDING  DEGENERATION  OF  THE  SPINAL  CORD 

When  as  a  result  of  an  injury  a  nerve-fiber  is  divided,  that  part  which  is 

severed  from  its  cell  of  origin  degenerates,  while  the  part  still  connected  with 

that  cell  usually  remains  intact.    This  is  known  as  Wallerian  degeneration,  and. 

as  will  be  readily  understood,  gives  valuable  information  concerning  the  course 


io6 


THE    NERVOUS    SYSTEM 


of  the  fiber  tracts.  In  case  of  a  complete  transection  of  the  spinal  cord  all  the 
ascending  fibers  whose  cells  are  located  below  the  cut  will  degenerate  in  the 
segments  above;  while  those  descending  fibers  whose  cells  of  origin  are  located 
above  will  degenerate  below  the  lesion  (Fig.  75).  Injury  to  the  dorsal  roots 
proximal  to  the  spinal  ganglia  causes  a  degeneration  of  the  dorsal  root  fibers 


Dorsal  spinocerebellar  trad 


Corticospinal  tract 


Ascending  branches  of  dorsal  root  fibers 


Fasciculus  proprius 

Descending  branch  of  dorsal  root  fiber 


Fig.  75. — Diagram  of  the  spinal  cord  to  illustrate  the  principle  of  Wallerian  degeneration. 
The  broken  lines  represent  the  degeneration  resulting  from— 1,  section  of  the  ventral  root;  2, 
section  of  the  spinal  nerve  distal  to  the  spinal  ganglion;  3,  section  of  the  dorsal  root  proximal  to 
the  spinal  ganglion,  and  4,  a  lesion  in  the  lateral  funiculus. 

throughout  their  length  in  the  spinal  cord.  Brain  injuries  may.  according  to 
their  location,  result  in  the  degeneration  of  one  or  more  of  the  tracts  which 
descend  into  the  spinal  cord  from  above. 

By  the  study  of  a  great  many  cases  of  injury  to  the  central  nervous  system 
in  man  and  of  experimentally  produced  lesions  in  animals  a  very  considerable 


I  H:i  l:     l  R  ICTS   01      mi     SPINAL    I  ORD 


107 


amount  of  information   has  been   obtained   concerning   the   fiber   tracts  of   the 
spinal  cord  (Collier  and  Buz/ard.  1901,  L903;  Stewart.  1901;  Thielc  and  Hoi 
1901;  Batten  and  Holmes,  1913).    This  is  summarized  in  the  accompanying  table 
and  in  Fig.  78. 

Tabu   Showing  thk  Location  of  the  Chief  Fibeb  Tracts  oi   mi   Spinal  Cobb  and  the 

1  >IKI  U    BON     IN     W  HII   II      I  111   S      I  >  I  <  .  I    N  I    |<  \  || 


Ascending  iletrcneration. 

liny  degeneration. 

Anterior  funiculus 

Ventral  spinothalamic  tract 

Ventral  corticospinal  tract, 
Vestibulospinal  tract, 
Tectospinal  tract 

Lateral  funiculus 

Dorsal  spinocerebellar  tract, 
Ventral  spinocerebellar  tract, 
Lateral  spinothalamic  tract, 
Spinotectal  tract 

Lateral  corticospinal  tract, 
Rubrospinal  tract, 
Bulbospinal  tract, 
Tectospinal  tract 

Posterior  funiculus 

Ascending  branches  of  the 
dorsal  root  fibers 

Fasciculus  interfascicularis, 
Septomarginal  tract 

The  fasciculi  proprii  or  ground  bundles  are  composed  of  short  ascending 
and  descending  fibers,  which  arise  and  terminate  within  the  gray  matter  of  the 
spinal  cord  and  link  together  the  various  segments  of  the  cord.  These  fascicles, 
one  of  which  is  present  in  each  of  the  three  funiculi,  immediately  surround 
the  gray  columns.  After  a  transection  of  the  spinal  cord  the  fasciculi  proprii 
undergo  an  incomplete  degeneration  for  some  distance  both  above  and  below 
the  lesion  (Figs.  75.  76).  In  cross-section  the  ground  bundle  of  the  posterior 
funiculus  has  the  form  of  a  narrow  band  upon  the  surface  of  the  posterior  column 
and  posterior  commissure,  and  was  once  called  the  cornu-commissural  bundle 
(Fig.  78).  In  addition  to  this  fascicle  there  are  in  the  posterior  funiculus  two 
other  tracts  which  in  part  belong  to  the  same  system — the  septomarginal  tract 
and  the  fasciculus  interfascicularis,  or  comma  tract  of  Schultze.  These  are 
both  composed  of  descending  fibers,  in  part  of  intraspinal  origin  and  in  part 
representing  the  descending  branches  of  the  dorsal  root  fibers.  The  septomar- 
ginal tract  is  situated  along  the  dorsal  periphery  of  the  posterior  funiculus  in 
the  thoracic  region;  it  takes  up  a  position  along  the  septum  in  the  lumbar  segments 
(oval  area  of  Flechsig);  and  in  the  sacral  region  it  forms  a  triangular  field  at  the 
dorsomedial  angle  of  the  posterior  funiculus  (triangle  of  Gombault  and  Philippe 
(Fig.  76).  The  fasciculus  interfascicularis  is  best  developed  in  the  thoracic 
segments,  where  it  occupies  a  position  near  the  center  of  the  posterior  funiculus. 


io8 


TILi:    NERVOUS    SYSTEM 


In  the  anterior  funiculus,  in  addition  to  the  fasciculus  proprius  which  imme- 
diately surrounds  the  gray  matter,  there  is  a  thin  layer  of  similar  fibers  spread 
out  along  the  border  of  the  anterior  fissure  and  known  as  the  sulcomarginal 
fasciculus.  This  tract  also  contains  the  fibers  which  descend  into  the  cord  from 
the  medial  longitudinal  bundle  of  the  medulla  oblongata. 

As  a  general  rule  the  short  fibers  of  the  fasciculus  proprius  lie  nearer  the 
gray  substance  than  the  fibers  of  greater  length;  and  the  long  tracts,  which 


Fasciculus  gracilis .. 


Spinocerebellar,  spinotectal,  and  lateral _ 
spinothalamic  tracts 


Cervical  enlargement 
ascending  degeneration 


I  'ppcr  thoracic 
ascending  degeneration 


'~M  Middle  thoracic 

.$      site  of  compression 


Lower  thoracic 

descending  degeneration 


Upper  lumbar 
descending  degeneration 


Lower  lumbar 

descending  degeneration 


Fig.  76. — Ascending  and  descending  degeneration  resulting  from  a  compression  of  the  thoracic 
spinal  cord  in  man.     March i  method.     (Hoche.) 

connect  the  spinal  cord  with  the  brain,  occupy  the  most  peripheral  position. 
But  the  fact  must  not  be  overlooked  that  many  fibers  of  the  fasciculus  proprius 
are  intermingled  with  those  of  the  long  tracts. 

LONG  DESCENDING  TRACTS  OF  THE  SPINAL  CORD 

Fibers  which  arise  from  cells  in  various  parts  of  the  brain  descend  into  the 
spinal  cord,  where  they  form  several  well-defined  tracts.     The  most  important 


Fasciculus  intcrfascicularis  «... 


Septomarginal  fasciculus-... 

Lateral  corticospinal  lracl~.j 

'■I 

mm 


Septomarginal  fasciculus,  oval  area  of  Flcchsig 

Lateral  corticospinal  tracU,^ 


I  ir.iK    TB  \<  is    OF   nil.    SPINAL   CORD 

and  most  conspicuous  of  these  are  the  cerebrospinal  fasciculi,  which  are  more 
properly  called  the  corticospinal  trails.  There  are  two  in  each  lateral  half  of 
the  cord,  the  lateral  and  the  ventral  corticospinal  tracts.  Their  cob  tituenl 
fibers  take  origin  from  the  Large  pyramidal  cells  of  the  precentral  gyrus  or  motor 
region  of  the  cerebral  cortex  and  pass  through  the  subjacent  Levels  of  the  brain 

to  reach  the  spinal  cord  (Fig.  77).      J  list  before  they  enter  the  spinal  COrd  they 

undergo  an  incomplete  decussation  in  the  medulla  oblongata,  giving  ri>e  to  a 
ventral  and  a  lateral  corticospinal  tract. 

The    Lateral    Corticospinal    Tract    (Crossed    Pyramidal    Trad,     Fasciculus 
Cerebrospinalis  Lateralis). — The  majority  of  the  pyramidal  libers,  after  cross 
ing  the  median  plane  in  the  decussation  of  the  pyramids,  enter  the  lateral  fu- 


Cerebral  hemisphere 


Spinal 
cord 


Fig.  77. — Diagram  of  the  corticospinal  tracts. 


niculus  of  the  spinal  cord  as  the  lateral  corticospinal  tract,  which  occupies  a  posi- 
tion between  the  dorsal  spinocerebellar  tract  and  the  lateral  fasciculus  proprius 
(Fig.  78).  In  the  lumbar  and  sacral  regions,  below  the  origin  of  the  dorsal 
spinocerebellar  tract,  the  lateral  corticospinal  tract  is  more  superficial.  It  can 
be  traced  as  a  distinct  strand  as  far  as  the  fourth  sacral  segment;  and  as  it 
descends  in  the  spinal  cord  it  gradually  decreases  in  size.  Throughout  its 
course  in  the  spinal  cord  it  gives  off  collateral  and  terminal  fibers  which  end  in 
the  gray  matter. 

The  ventral  corticospinal  tract  (fasciculus  cerebrospinalis  anterior  or  direct 
pyramidal  tract)  is  formed  by  the  smaller  part  of  the  corticospinal  fibers,  which 
do  not  cross  in  the  medulla,  but  pass  directly  into  the  ventral  funiculus  of  the 


no 


THE    NERVOUS    SYSTEM 


same  side  of  the  cord.  They  form  a  tract  of  small  size,  which  lies  near  the 
anterior  median  fissure  and  which  can  be  traced  as  a  distinct  strand  as  far  as  the 
middle  of  the  thoracic  region  of  the  spinal  cord.  Just  before  terminating  these 
fibers  cross  in  the  anterior  white  commissure.  They  end  like  those  of  the  lateral 
corticospinal  tract,  either  directly  or  perhaps  through  an  intercalated  neuron, 
in  relation  to  the  motor  cells  in  the  anterior  column.  The  crossing  of  these 
libers  is  only  delayed,  and  it  will  be  apparent  that  all  of  the  corticospinal  fibers 
arising  in  the  right  cerebral  hemisphere  terminate  in  the  anterior  column  of  the 
left  side  of  the  cord,  and  conversely,  those  from  the  left  hemisphere  end  on  the 
right  side.  It  is  along  these  fibers  that  impulses  from  the  motor  portion  of  the 
cerebral  cortex  reach  the  cord  and  bring  the  spinal  motor  apparatus  under 
voluntarv  control. 


Fasciculus  septomarginalis 


Fasciculus  gracilis 


Fasciculus  inlerfasciciilaris 

Fascicillus  proprius  , 

Sensory  fibers  of  the 
second  order 
Lateral  corticospinal _ 
tract 

Rubrospinal  tract— 

Tectospinal  tract  - 

Fasciculus  proprius- 

Bulbospinal  trad  — 
Vestibulospinal  tract 


,,-'Fasciculus  cuneatus 


-Dorsolateral  fasciculus 

_^s  Dorsal  spinocerebellar 
trad 

\      Fasciculus  proprius 

Ventral  spinocere- 
bellar trad 
'...Lateral  spinothalamic 
tract 
"  Spinotectal  trad 

—  Ventral  root 

""  Ventral  spinothalamic  trad 

Sulcomarginal  fasciculus 
Ventral  corticospinal  trad 

Fig.  78. — Diagram  showing  the  location  of  the  principal  fiber  tracts  in  the  spinal  cord  of  man. 
Ascending  tracts  on  the  right  side,  descending  tracts  on  the  left. 

It  is  stated  by  some  authors,  although  on  the  basis  of  rather  unsatisfactory  evidence, 
that  the  fibers  of  the  lateral  corticospinal  tract  ramify  in  the  formatio  reticularis  (Mona- 
kow.  1895)  and  the  nucleus  dorsalis  (Schafer,  1899).  The  corticospinal  path  is  from  the 
standpoint  of  phylogenesis  a  relatively  new  system  and  varies  a  great  deal  in  different 
mammals.  It  is  found  in  the  ventral  funiculus  in  the  mole,  while  in  the  rat  it  occupies  the 
posterior  funiculus.  In  the  mole  it  is  almost  completely  unmyelinated,  in  the  rat  largely  so. 
It  contains  many  unmyelinated  fibers  in  the  cat,  fewer  in  the  monkey  (Linowiecki,  1914). 
In  man  it  does  not  become  fully  myelinated  before  the  second  year.  An  uncrossed  ventral 
corticospinal  tract  seems  to  be  present  only  in  man  and  the  anthropoid  apes,  and  this  tract 
varies  greatly  in  size  in  different  individuals. 

The  rubrospinal  tract  (tract  of  Monakow)  is  situated  near  the  center  of  the 
lateral  funiculus  just  ventral  to  the  lateral  corticospinal  tract  (Fig.  78).  Its 
fibers  come  from  the  red  nucleus  of  the  mesencephalon,  cross  the  median  plane, 


)  ir.l  R     CRACTS    OF    Nil.    SPDN  \l.   I  ORD  i  i  | 

and  descend  Into  the  spinal  cord,  within  which  some  of  them  can  be  traced  to 
the  sacral  region.  Their  collateral  and  terminal  branches  end  within  the  an- 
terior column  in  relation  to  the  primary  motor  neurons. 

Other  Descending  Tracts.  The  bulbospinal  tract  (olivospinal  tract,  trad  of 
Helweg)  is  a  small  bundle  of  fibers  found  in  the  cervical  region  near  the  surface 
of  the  lateral  funiculus  opposite  the  anterior  column.  The  fibers  arise  from 
cells  in  the  medulla  oblongata,  possibly  in  the  inferior  olivary  nucleus,  and  end 
somewhere  in  the  gray  matter  of  the  spinal  cord.     The  exact  origin  and  u-r- 

Fasciculus  cuneatus  Fasciculus  gracilis 


Lateral  corticospinal  tract 


Fasciculi  proprii<\ 


Ventral  corticospinal  tract 


'   '  Dorsal  spinocerebellar  tract 


Oral  area  of  Flcchsig 


D.  Ill 


L.  IV 


Fig.  80. 
Figs.  79  and  80. — Diagrams  of  the  sixth  cervical,  third  thoracic,  and  fourth  lumbar  segments 
of  the  spinal  cord,  showing  the  location  of  the  different  tracts  as  outlined  by  Flechsig  on  the  basis 
of  differences  in  time  of  myelination.      (van  Gehuchten.) 


ruination  of  the  tract  is  unknown.  The  tectospinal  tract,  located  in  the  ventral 
funiculus,  is  composed  of  fibers  which  take  origin  in  the  roof  (tectum)  of  the 
mesencephalon,  cross  the  median  plane  and  descend  into  the  anterior  funiculus 
of  the  spinal  cord,  and  end  in  the  gray  matter  of  the  anterior  column.  The  tract 
is  concerned  chiefly  with  optic  reflexes.  The  vestibulospinal  tract,  also  located 
in  the  anterior  funiculus,  arises  from  the  lateral  nucleus  of  the  vestibular  nerve 


112  THE    NERVOUS    SYSTEM 

in  the  medulla  oblongata  and  conveys  impulses  concerned  in  the  maintenance 
of  equilibrium.  Some  of  its  fibers  can  be  traced  as  far  as  the  lower  lumbar 
segments.     They  end  in  the  gray  matter  of  the  anterior  column. 

Hemisection  of  the  spinal  cord  in  man  produces  a  characteristic  symptom 
complex  known  as  the  Brown-Sequard's  syndrome — which  the  student  is  now  in 
position  to  understand.  Below  the  level  of  the  lesion  and  on  the  same  side 
there  is  found  a  paralysis  of  the  muscles  with  a  loss  of  sensation  from  the  mus- 
cles, joints,  and  tendons;  while  on  the  opposite  side  of  the  body,  beginning  two 
or  three  segments  below  the  level  of  the  lesion,  there  is  loss  of  sensations  of 
pain  and  temperature.  Tactile  sensibility  is  everywhere  retained  (Dejerine, 
1914). 

Order  of  Myelination. — The  fiber  tracts  of  the  spinal  cord  do  not  all  become 
myelinated  at  the  same  time.  By  a  study  of  the  fetal  spinal  cord  at  various 
developmental  stages  Flechsig  was  able  to  identify  and  trace  many  of  these 
tracts  because  of  the  difference  in  the  time  of  myelination.  His  results  agree 
in  general  with  those  derived  from  a  study  of  spinal  cords  showing  ascending 
and  descending  degeneration  (Figs.  79.  80).  Myelination  begins  during  the  fifth 
month  of  intra-uterine  life.  The  order  in  which  the  fibers  of  the  spinal  cord 
acquire  their  myelin  sheaths  is  as  follows:  (1)  afferent  and  efferent  root  fibers, 
(2)  those  of  the  fasciculi  proprii.  (3)  the  fasciculus  cuneatus,  (4)  the  fasciculus 
gracilis,  (5)  the  dorsal  spinocerebellar  tract,  (6)  the  ventral  spinocerebellar  fas- 
ciculus, (7)  the  corticospinal  tracts. 


CHAPTER  VIII 

THE  GENERAL  TOPOGRAPHY  OF  THE  BRAIN.  THE  EXTERNAL 
FORM  OF  THE  MEDULLA  OBLONGATA,  PONS,  AND  MESEN- 
CEPHALON 

The  General  Topography  of  the  Brain. — The  brain  rests  upon  the  floor  of 
the  cranial  cavity,  which  presents  three  well-marked  fossae.  In  the  posterior 
cranial  fossa  are  lodged  the  medulla  oblongata,  pons,  and  cerebellum,  which 
together' constitute  the  rhombencephalon  (Fig.  81).  This  fossa  is  roofed  over 
by  a  partition  of  dura  mater,  called  the  tentorium  cerebetti,  that  separates  the 
cerebellum  from  the  cerebral  hemispheres.     Through  the  notch  in  the  ventral 


Calvaria 


Proscn-   Telencephalon 
cephalon  Diencephalon 

Frontal    lobe    of    cerebral 

hemisphere    in    anterior 
cranial  fossa 
Temporal  lobe  of  cerebral 
hemisphere    in     middle 
cranial  fossa 


Parietal    lobe    of    cert 
hemisphere 


Mesencephalon 


Occipital  lobe    of  cerebral 

hemisphere 
Tentorium  cerebetti 
Posterior  cranial  fossa 

Hum 

Pons 
Medulla  oblot 
Spinal  cord 


Fig.  81. — Median  sagittal  section  of  the  head  showing  the  relation  of  the-  brain  to  the  cra- 
nium. The  sphenoid  bone  is  shown  in  transparency,  and  through  it  the  temporal  lobe  may  be 
seen. 

border  of  the  tentorium  projects  the  mesencephalon,  connecting  the  rhomben- 
cephalon below  with  the  prosencephalon  above  that  partition.  The  cerebral 
hemispheres  form  the  largest  part  of  the  prosencephalon,  occupy  the  anterior 
and  middle  cranial  fossa?,  and  extend  to  the  occiput  on  the  upper  surface  of  the 
tentorium. 

The  dorsal  aspect  of  the  human  brain  presents  an  ovoid  figure,  the  large 
cerebral  hemispheres,  covering  the  other  parts  from  view.     In  the  sheep's  brain  the 
8  "3 


1 1.1 


THE    NERVOUS    SYSTEM 


hemispheres  are  smaller  and  fail  to  hide  the  cerebellum  and  medulla  oblongata 
(Fig.  82).  The  cerebral  hemispheres,  which  are  separated  by  a  deep  cleft  called 
the  longitudinal  fissure  of  the  cerebrum,  together  present  a  broad  convex  surface 
which  lies  in  close  relation  to  the  internal  aspect  of  the  calvaria.  From  the 
latter  it  is  separated  only  by  the  investing  membranes  or  meninges  of  the  brain. 
The  thin  convoluted  layer  of  gray  matter  upon  the  surface  of  the  hemispheres  is 
known  as  the  cerebral  cortex. 

The  ventral  aspect  or  base  of  the  brain  presents  an  irregular  surface  adapted 
to  the  uneven  floor  of  the  cranial  cavity  (Figs.  83,  86).     The  medulla  oblongata, 


Face  and  tongue 

Head  and  eyes 

Fore  limb 

Hind  limb 

Gyrus  sylviacus  (arcuatus) 

Gyrus  lateralis 
Gyri  mediates  -^ 

Gvrus  intemus  _ 


Vermis  cerebelli 
Hemisphcerium  cerebelli 

Medulla  oblongata 

Medulla  spinalis 


Gyrus  frontalis  medial  is 
Gyrus  frontalis  superior 
Sulcus  coronalis 
Sulcus  splenialis 

Fissura  ansata  (cruciala) 
Fissura  lateralis  (Sylvii) 

Fissura  suprasylvia 
Fissura  longitudinalis 
Sulcus  lateralis 
Sulcus  intermedins 
Sulcus  medialis 


Flocculus 

-  —  Nervus  accessorius 

Nervus  spinalis  I 


Fig.  82. — Dorsal  view  of  the  sheep's  brain.     The  motor  cortex  is  shaded  on  the  left  side.     (Herrick 

and  Crosby.) 


which  is  continuous  through  the  foramen  magnum  with  the  spinal  cord,  lies  on 
the  ventral  aspect  of  the  cerebellum  in  the  vallecula  between  the  two  cere- 
bellar hemispheres.  Rostral  to  the  medulla  oblongata  and  separated  from  it 
only  by  a  transverse  groove  is  a  broad  elevated  band  of  fibers,  which  plunges 
into  the  cerebellum  on  either  side  and  is  known  as  the  pons.  The  cerebellum 
can  be  seen  occupying  a  position  dorsal  to  the  pons  and  medulla  oblongata,  and 
can  easily  be  recognized  by  its  grayish  color  and  many  parallel  fissures.  A 
pair  of  large  rope-like  strands  are  seen  to  emerge  from  the  rostral  border  of 
the  pons  and  to  diverge  from  each  other  as  they  run  toward  the  under  surface 


111!     GENERAL    I'  IPl  IGB  \l'll\    I  'I      I  II!      BR  W\ 


i  IS 


of  the  cerebral  hemispheres.     These  are  the  cerebral  peduncles  and  they  form 
the  ventral  part  of  the  mesencephalon.    At  it-  rostral  extremit)  each  peduncle 
i>  partially  encircled  by  a  flattened  hand,  known  as  the  optic  tract,  which  is  con 
tinuous  through  the  optic  chiasma  with  the  optic  nerves.     A  lozenge-shaped 

depression,  known  as  the  interpeduncular  fossa,  i>  outlined  by  the  diverging 
cerebral  peduncles  and  by  the  optic  chiasma  and  tract-.  Within  the  area  thu> 
outlined  and  beginning  at  its  caudal  angle  may  he  distinguished  the  following 

parts:    the  inter  peduncular  nucleus,  which  i-  very  large  in  the  sheep  and  OCCU- 


Longitudinal  fissure  of  cerebrum 
Optic  lurve  _ 
Optic  chiasma 
Rkinal  fissurt 

Insula--^ 
Lateral  fissure 

Optic  tract  . 

Infundibulum  -■ 

Manimillary  body  - 

Cerebral  pedunch 
Interpeduncular  fossa  and 
nucleus 

Trigeminal  nerve 

Abduccns  nerve-- 

.        ,.  [Vestibular  n 
Acoustie] 

[Cochlear  n. 

Glossopharyngeal  tier:'  --" 

Vagus  nerve'' 

Hypoglossal  nerve  ■' 

Anterior  median  fissurt 


\'  Olfactory  bulb 

edial  olfactory  gyrus 
interior  perforated  •uhstance 
Lateral  olfactory  stria 
■—'Lateral  olfactory  gyrus 
-—Diagonal  band 

Amygdaloid  nucleus 

Pyriform  <irea 
~'j—  Hippocampal  gyrus 
--   Trochlear  nerce 
,/--- Pons 
__.--  Abduccns  nerve 
/ _.--  Facial  nerve 

- '  Trapezoid  body 
Cerebellum 
'--Olive 
"-Chorioid  plexus 
<ory  nerve 
KTractus  lateralis  minor 


Fig.  83. — Ventral  view  of  the  sheep's  brain. 

pies  an  area  designated  in  man  as  the  substantia  perforata  posterior;  the  corpus 
mammillare,  which  in  man  is  divided  by  a  longitudinal  groove  into  two  mani- 
millary bodies;  and  also  the  tuber  cinereum,  infundibulum,  and  hypophysis. 
Rostral  to  the  optic  tract  there  is  on  either  side  a  triangular  field  of  gray  matter, 
studded  with  minute  pit-like  depressions  and  known  as  the  anterior  perforated 
substance. 

The  Rhinencephalon .— The  olfactory  bulb  is  situated  near  the  rostral  end 
of  the  hemisphere,  to  the  ventral  surface  of  which  it  is  attached  by  the  olfactory 


u6 


THE    NERVOUS    SYSTEM 


peduncle  'and  in  man  by  the  long  olfactory  tract).  In  the  sheep's  brain  there 
diverge  from  the  olfactory  peduncle  two  well-defined  gray  bands,  the  medial 
and  lateral  olfactory  gyri.  which  are  less  evident  in  man;  and  furthermore,  the 
lateral  olfactory  gyrus  is  obviously  continuous  with  the  hippocampal  gyrus. 
forming  the  pyrijorm  area  (Fig.  83).  All  of  these  structures  are  closely  asso- 
ciated in  function  and  belong  to  the  rhinencephalon,  or  olfactory  part  of  the 
brain,  which,  because  of  the  greater  importance  of  the  sense  of  smell  in  the 
sheep,  is  better  developed  in  that  animal  than  in  man.  A  prominent  longi- 
tudinal fissure  separates  this  part  of  the  brain  from  the  rest  of  the  hemisphere. 

Interventricular  foramen    Body  of  corpus  callosum 
Anterior  commissure . 
Septum  pcllucidum 
Rostral  lamina    ' 
Rostrum  of  corpus  callosum  • 
Genu  of  corpus  callosum 


Body  of  fornix 


'  i   i    '    Hippocampal  com.  Roofs  of  third  ventricle  or  tela  chorioidca 
\\  \  •   \   Stria  med.  /Haben  com. 

1  \  \  \  \  '.    Habcnular      •'  > Splenium  Suprapineal  recess 


Suprapi 
/  .Superior  colliculus 
/   /Primary  fissurr 

White  center  of  v<  rmis 


Infundib. 


■Pons 
Aqueduct 


Olfactory  bulb 

Medial  olfaclor'y  gyrus;,,      ;  ;        Tu}d  ^  ^  N  >  La;nina        ^ 

Anterior  per],  substance  >  ,        /  <  -. ,          .   .          ,.  x  \    p„c,„  •, *„„, 

Lamina  terminalis'/       /  f^assa  intermedia  \\^«JSZ 

Diagonal  band         1  Optic  cktasma  ^Hypophysis 


Central  canal 
,  "  Medulla 
\   Medial  aperture   of 
\  \     fourth  ventricle 
\  \Tela  chorioidca 
\   "  Fourth  ventricle 
x  A  nterinr  medullary 
velum 


Preoptic  recess  Mammittary  body 

Fig.  84. — Medial  sagittal  section  of  the  sheep's  brain. 

This  is  known  as  the  rhinal  fissure;  and  all  that  portion  of  the  cerebral  cortex 
which  lies  dorsal  to  it  i>  the  new  or  non-olfactory  cortex,  the  neopallium.  In 
contrast  to  the  older  olfactory  cortex  or  archi pallium,  which  includes  the  p\Ti- 
form  area,  the  neopallium  is  of  recent  phyletic  development.  It  first  forms  a 
prominent  part  of  the  brain  in  mammals  and  is  by  far  the  most  highly  developed 
in  man. 

Interrelation  of  the  Various  Parts  of  the  Brain.— An  examination  of  a  medial 
sagittal  section  of  the  brain  will  make  clear  the  relation  which  the  various  parts 
bear  to  each  other  (Fig.  84).  The  medulla  oblongata,  pons,  and  cerebellum  are 
seen  surrounding  the  fourth  ventricle,  and  are  intimately  connected  with  one 


THE    GENERA]     TOPOGRAPHS    OF    I  UK    BRAIN  117 

another.  The  medulla  oblongata  is  directly  continuous  with  the  pons,  and  on 
either  side  a  large  bundle  of  libers  from  the  dorsal  aspec  t  of  the  former  runs  into 
the  cerebellum.  These  two  strands,  which  are  known  as  the  restiform  bodies 
or  inferior  cerebellar  peduncles,  constitute  the  chief  avenues  of  communication 
between  the  spinal  cord  and  medulla  oblongata  on  the  one  band  and  the  cere 
bellum  on  the  other.  The  ventral  prominence  of  the-  pons  is  produced  in  large 
part  by  transverse  bundles  of  libers,  which  when  traced  lateralward  are  seen  to 
form  a  large  strand,  the  brachium  pontis  or  middle  cerebellar  peduncle,  that 
enters  the  corresponding  cerebellar  hemisphere  (Figs.  83,  86).  The  brachium 
conjunctivum  or  superior  cerebellar  peduncle  can  be  traced  ro^t rally  from  the 
cerebellum  to  the  mesencephalon.  The  three  peduncles  are  paired  structures, 
symmetrically  placed  on  the  two  sides  of  the  brain  (Figs.  87,  88). 

The  Cerebrum. — The  mesencephalon  surrounds  the  cerebral  aqueduct  and 
consists  of  the  ventrally  placed  cerebral  peduncles,  and  a  dorsal  plate  with  four 
rounded  elevations,  the  lamina  and  corpora  quadrigemina  (superior  and  inferior 
colliculi).  The  cerebral  hemispheres  form  the  most  prominent  part  of  the 
cerebrum  and  are  separated  from  each  other  by  the  longitudinal  fissure  (Fig. 
82),  at  the  bottom  of  which  is  a  broad  commissural  band,  the  corpus  callosum, 
which  joins  the  two  hemispheres  together  (Fig.  85).  Under  cover  of  the  cere- 
bral hemispheres  and  concealed  by  them,  except  on  the  ventral  aspect  of  the 
brain,  is  the  diencephalon.  This  includes  most  of  the  parts  which  help  to  form 
the  walls  of  the  third  ventricle.  These  are  from  above  downward,  the  c  pi  thal- 
amus, including  the  habenular  trigone  and  pineal  body  near  the  roof  of  the 
ventricle;  the  thalamus,  which  forms  most  of  the  lateral  wall  of  the  ventricle, 
and  is  united  with  its  fellow  across  the  cavity  by  a  short  bar  of  gray  substance, 
the  massa  intermedia;  and  the  hypothalamus,  including  the  mammillary  bodies, 
infundibulum,  and  part  of  the  hypophysis  (Figs.  84,  85). 

The  Brain  Ventricles.— The  central  canal  of  the  spinal  cord  is  prolonged 
through  the  caudal  portion  of  the  medulla  oblongata  and  finally  opens  out  into 
the  broad  rhomboidal  fourth  ventricle  of  the  rhombencephalon.  At  its  pointed 
rostral  extremity  this  ventricle  is  continuous  with  the  cerebral  aqueduct,  the 
elongated  slender  cavity  of  the  mesencephalon.  This,  in  turn,  opens  into  the 
third  ventricle,  which  is  a  narrow  vertical  cleft  between  the  two  laterally  sym- 
metric halves  of  the  diencephalon.  It  is  bridged  by  the  massa  intermedia. 
Near  the  dorsal  part  of  the  rostral  border  of  the  ventricle  is  a  small  opening  in 
each  lateral  wall,  the  interventricular  foramen  or  foramen  of  Monro.  I  his 
leads  into  the  lateral  ventricle,  the  cavity  of  the  cerebral  hemisphere. 


ii8  the  nervous  system 

THE  ANATDMY  OF  THE  MEDULLA  OBLONGATA 

At  its  rostral  end  the  spinal  cord  increases  in  size  and  goes  over  without 
sharp  line  of  demarcation  into  the  medulla  oblongata,  or  myelencephalon,  which, 
as  we  learned  in  Chapter  II,  is  derived  from  the  posterior  part  of  the  third  brain 
vesicle.  The  medulla  oblongata  may  be  said  to  begin  just  rostral  to  the  high- 
est rootlet  of  the  first  cervical  nerve  at  about  the  level  of  the  foramen  magnum; 


Marginal  fart  of  sulcus  cinguli 
Sulcus  of  corpus  callosum 
Splenium  of  corpus  callosum        \ 
Precuneus 
Sub  parietal  sulcus  . 
Paricto-occi  pita!  fissure 

Lamina  quadrigemina 

Cuneus 

Superior  vermis  % 


Calcarinc  fissure''. 
Occipital  pole 
Lingual  gyrus 
Tra  nsverse  fissure ' 
Cerebellar  hem 
Mid nllary  subsja 
of  vermis 

Inferior  vermis 
Calamus  scriptorius 
Central  canal 
Spinal  cord  ' 
Tela  chorioidea  of  fourth  ventricle 

Fourth  ventricle 
Medulla  oblongata 
Anterior  medullary  velum 

Cerebral  aqueduct 
Pons 
Posterior  perforated  substance 

Oculomotor  nerve 


Central  sulcus  in  paracentral  lobule 
Pineal  body 
Pineal  recess 

Posterior  commissure 

Tela  chorioidea  of  third  ventricle 
Massa  intermedia 
Gyrus  cinguli 
Thalamus 

Body  of  corpus  callosum 
■  Body  of  fornix 

Septum  pcllucidum 
Sulcus  cinguli 
Interventric.  foramen 
Column  of  fornix 
■Anterior  commis- 


;>  Superior  frontal 
gyrus 


'Frontal  pole 
Genu  of  corpus  callosum 
^Rostrum  of  cor  p.  callosum 
\\\\„^ Parol  factory  area  and  sulci 
\  \  \\  ^Subcallosal  gyrus 
K\  \  \  "Hypothalamic  sulcus 
\\  \  "Lamina  tcrminalis 
\  \  \  '  Optic  recess 
Optic  nerve 
Optic  chiasma 
Infundibulum 
Anterior  lobe   TT    .    . , 
Posterior  lobe  \HyP°Phsis 
Mammillary  body 


Fig.  85. — Medial  sagittal  section  of  the  human  brain.      (Sobotta-McMurrich.) 


and  at  the  opposite  extremity  it  is  separated  from  the  pons  by  a  horizontal  groove 
(Figs.  81,  85).  Its  ventral  surface  rests  upon  the  basilar  portion  of  the  occipital 
bone;  while  its  dorsal  surface  is  in  large  part  covered  by  the  cerebellum.  The 
shape  of  the  medulla  oblongata  is  roughly  that  of  a  truncated  cone,  the  smaller 
end  of  which  is  directed  caudally  and  is  continuous  with  the  spinal  cord.  In 
man  it  measures  about  3  cm.,  or  a  little  more  than  1  inch,  in  length  (Fig.  86). 
Like  the  spinal  cord,  the  medulla  oblongata  presents  a  number  of  more  or 


A\  \Im\1\    01     MM      Ml  M   II. \    0BL0NGA1  V 

less  parallel  longitudinal  grooves.  These  arc  the  anterior  and  posterior  median 
fissures,  and  a  pair  each  <>i'  anterior  lateral  and  posterior  lateral  sulci  I 
89).  By  means  of  tin-  fissures  it  i>  divided  symmetricallj  into  righl  and  left 
halves;  while  these,  in  turn,  an-  marked  off  by  the  sulci  into  ventral,  lateral,  ami 
dorsal  areas,  which  as  seen  from  the  surface  appear  to  be  the  direct  upward  con- 
tinuation  of  tin-  anterior,  lateral,  and  posterior  funiculi  of  the  spinal  cord. 
But,  as  we  shall  sec  in  the  following  chapter,  this  continuity  is  not  as  perfeel 
a-  it  appears  from  the  surface;  because  tin-  tracts  <>f  the  cord  undergo  a  rear- 
rangement as  they  enter  the  medulla  oblongata.  The  posterior  median  fissuM 
does  not  extend  beyond  the  middle  of  the  oblongata,  at  which  point  its  lips 
separate  to  form  the  lateral  boundaries  of  the  caudal  portion  of  tin-  fourth  ven- 
tricle. The  caudal  half  of  the  medulla  oblongata  contains  a  canal,  the  dire<  t 
continuation  of  the  central  canal  of  the  spinal  cord,  and  is  known  as  the  dosed 
portion  of  the  medulla  oblongata  (Fig.  85).  This  canal  opens  out  into  the  fourth 
ventricle  in  the  rostral  half,  which  helps  to  form  the  ventricular  floor,  and  which 
is  often  spoken  of  as  the  open  part  of  the  medulla  oblongata. 

Fissures  and  Sulci. — The  posterior  median  fissure  represents  the  continua- 
tion of  the  posterior  median  sulcus  of  the  spinal  cord  and,  as  noted  above,  ends 
near  the  middle  of  the  medulla  oblongata.  The  anterior  median  fissure  is  con- 
tinued from  the  spinal  cord  to  the  border  of  the  pons,  where  it  ends  abruptly 
in  a  pit  known  as  the  foramen  ccBcum.  Near  the  caudal  extremity  of  the  medulla 
oblongata  this  fissure  is  interrupted  by  interdigitating  bundles  of  fibers  which 
pass  obliquely  across  the  median  plane.  These  are  the  fibers  of  the  lateral 
corticospinal  tract,  which  undergo  a  decussation  on  passing  from  the  medulla 
oblongata  into  the  spinal  cord,  known  as  the  decussation  of  the  pyramids.  The 
anterior  lateral  sulcus  also  extends  throughout  the  length  of  the  medulla  ob- 
longata and  represents  the  upward  continuation  of  a  much  more  indefinite  groove 
bearing  the  same  name  in  the  spinal  cord.  From  it  emerge  the  root  filaments 
of  the  hypoglossal  nerve.  From  the  posterior  lateral  sulcus  emerge  the  rootlets 
of  the  glossopharyngeal,  vagus,  and  accessory  nerves  (Figs.  86,  88,  89). 

The  ventral  area  of  the  medulla  oblongata  is  included  between  the  anterior 
median  fissure  and  the  anterior  lateral  sulcus,  and  has  the  false  appearance  of 
being  a  direct  continuation  of  the  anterior  funiculus  of  the  spinal  cord.  On 
either  side  of  the  anterior  median  fissure  there  is  an  elongated  eminence,  taper- 
ing toward  the  spinal  cord,  and  known  as  the  pyramid  (pyramis — Fig.  86).  It 
is  formed  by  the  fibers  of  the  corticospinal  or  pyramidal  tract.  Just  before  the 
fibers  of  this  tract  enter  the  spinal  cord  they  undergo  a  more  or  less  complete 


120 


THE    NERVOUS    SYSTEM 


decussation,  crossing  the  median  plane  in  large  obliquely  interdigitating  bundles, 
which  nil  up  and  almost  obliterate  the  anterior  median  fissure  in  the  caudal 
part  of  the  medulla  oblongata.  This  is  known  as  the  decussation  of  the  pyra- 
mids (decussatio  pyramidum).  In  the  sheep  these  fibers  pass  into  the  opposite 
posterior  funiculus  of  the  spinal  cord.     In  man  the  crossing  is  incomplete,  a 


Infundibulum 

Orbital  sulci  of  frontal  lobe 
Orbital  gyri  of  frontal  lobe 


Hypophyi 

Temporal  pole 

Anterior  pcrfor.  substance 

Oculomotor  nerve  -^  I 

Uncus  -.  '£ 

Mammillary  body  v    f1 

Cerebral   peduncle  -  ;  v 

Pons  rrn 

1  ngcminal  nerve  .  J  / 
Temporal  lobe  ^Mgr^ 
Facial  nerve  Jb 


frontal  pole     olfartory  sukus 

Olfactory  bulb 
,  Olfactory  tract 
Optic  nerve 


*  >r 


i    Optic  chiasma 

ra^^-  Lateral  olfactory  stria 

Fm  .-  Tuber  cincrcum 
j^^^^k.   Maxillary  nerve 

Ophthalmic  nerve 
Portio  minor  of  trigem. 
nerve 
Mandibular  nerve 

Semilunar  ganglion 

L--  Trochlear  nerve 


Ncrvus  intermedins-' 


Acouslic  nerve.' 


Flocculus  of  cerebellum ''^SjlR< 
Cerebellum     ^5v 

Chorioid  plexus  of  ventricle  IV     \ 
Glossopharyngeal  nerve  >' 


mi 


Vagus  nerve -'^^M 
Hypoglossal  nerve  -^^BBHI 
Accessory  nerve '      : 
Root  filaments  of  cervical  nerve  I 

Decussation  of  pyramids 


/iter peduncular  fossa 


Abducens  nerve 
Olive 
Pyramid 

Medulla  oblongata 
,      \  Tonsil  of  cerebellum 

\     Occipital  pole 
Spinal  cord 
Vermis  of  cerebellum 


Fig.  86. — Ventral  view  of  the  human  brain.     (Sobotta-McMurrich.) 


majority  of  the  fibers  descending  into  the  lateral  funiculus  of  the  opposite  side, 
a  minority  into  the  anterior  funiculus  of  the  same  side  (Fig.  77).  We  are  al- 
ready acquainted  with  these  bundles  in  the  spinal  cord  as  the  ventral  and  lateral 
corticospinal  tracts  (direct  and  crossed  pyramidal  tracts).  In  addition  to  the 
pyramid  the  ventral  area  of  the  medulla  also  contains  a  bundle  of  fibers,  the 


W  \1"\1Y    01     mi.    Ml  .Di  l.l.\    OBLONGATA 

medial  longitudinal  fasciculus ;  which  is  continuous  with  the  anterior  fasciculus 
proprius  of  the  spinal  cord. 

The  lateral  area  of  the  medulla  oblongata,  included  between  the  antero 
lateral  and  posterolateral  sulci,  appears  as  a  dired  continuation  of  the  lateral 
funiculus  of  the  spinal  cord;  hut,  as  a  matter  <>!'  hut,  many  of  the  fibers  of  that 
funiculus  find  their  way  into  the  anterior  area  (as.  lor  example,  the  lateral  cor- 
ticospinal tract)  or  into  the  posterior  area  (dorsal  spinocerebellar  tract).  In 
the  rostral  part  of  the  lateral  area,  between  the  root  filament-,  of  the  gli 
pharyngeal  and  vagus  nerves,  on  the  one  hand,  and  those  of  the  hypoglossal, 
on  the  other,  i>  an  oval  eminence,  the  olive  (oliva,  olivary  body),  which  is  pro- 
duced l>y  a  large  Irregular  mass  of  gray  substance,  the  inferior  olivary  nucleus, 
located  just  beneath  the  surface  (Figs.  87,  88).  By  a  careful  inspection  of  the 
surface  of  the  medulla  oblongata  it  is  possible  to  distinguish  numerous  fine 
bundles  of  fibers,  which  emerge  from  the  anterior  median  fissure  or  from  the 
groove  between  the  pyramid  and  the  olive  and  run  dorsally  upon  the  surface 
of  the  medulla  to  enter  the  restiform  bodies.  These  are  the  ventral  external 
arcuate  fibers  and  are  most  conspicuous  on  the  surface  of  the  olive  (Fig.  88). 

In  the  sheep  there  are  two  superficial  bands  of  fibers  not  seen  in  the  human 
brain.  Placed  transversely  near  the  caudal  border  of  the  pons  is  a  belt-like 
elevation,  known  as  the  trapezoid  body,  through  which  emerge  the  roots  of  the 
abducens  and  facial  nerves  (Figs.  83,  87).  In  man  the  much  larger  pons  cover.-, 
this  band  from  view  and  the  sixth  and  seventh  nerves  emerge  from  under  the 
caudal  border  of  the  pons.  Another  bundle,  beginning  on  the  ventral  sur- 
face of  the  trapezoid  body  near  the  seventh  nerve,  describes  a  graceful  curve 
around  the  ventral  border  of  the  olive  and  becomes  lost  in  the  lateral  area  of 
the  medulla  oblongata.     This  has  been  called  the  fasciculus  lateralis  minor. 

The  dorsal  area  of  the  medulla  oblongata  is  bounded  ventrally  by  the  pos- 
terolateral sulcus  and  emergent  root  filaments  of  the  glossopharyngeal,  vagus, 
and  accessory  nerves.  In  the  closed  part  of  the  medulla  oblongata  it  extends 
to  the  posterior  median  fissure,  while  in  the  open  part  its  dorsal  boundary  is 
formed  by  the  lateral  margin  of  the  floor  of  the  fourth  ventricle.  The  caudal 
portion  of  this  area  is,  in  reality,  as  it  appears,  the  direct  continuation  of  the 
posterior  funiculus  of  the  spinal  cord.  On  the  dorsal  aspect  of  the  medulla 
oblongata  the  fasciculus  cuneatus  and  fasciculus  gracilis  of  the  cord  are  con- 
tinued as  the  funiculus  cuneatus  and  funiculus  gracilis,  which  soon  enlarge  into 
elongated  eminences,  known  respectively  as  the  cuneate  tubercle  and  the  clava 
(Figs.  89,  91).     These  enlargements  are  produced  by  gray  masses,  the  nucleus 


122 


THE    NERVOUS    SYSTEM 


gracilis  and  nucleus  cunealus,  within  which  end  the  fibers  of  the  corresponding 
fasciculi  of  the  spinal  cord.  'J  he  clava  and  cuneate  tubercle  are  displaced  lat- 
erally by  the  caudal  angle  of  the  fourth  ventricle.  Somewhat  rostral  to  the  mid- 
dle of  the  medulla  oblongata  they  gradually  give  place  to  the  restiform  body. 

More  laterally,  between  the  cuneate  funiculus  and  tubercle  on  the  one  hand 
and  the  roots  of  the  glossopharyngeal,  vagus,  and  accessory  nerves  on  the  other, 
is  a  third  longitudinal  club-shaped  elevation  called  the  tuberculum  cinereum. 
It  is  produced  by  a  tract  of  descending  fibers,  derived  from  the  sensory  root  of 
the  trigeminal  nerve,  and  by  an  elongated  mass  of  substantia  gelatinosa  which 


Corona  radiata  -j 

Lentiform  nucleus 

Lateral  geniculate  body 

Medial  geniculate  body  •, 

Optic  radiation  •, 

Corona  radiata  , 

Pulvinar\s 

Inferior  quadrigeminal  brachium\ 

Superior  colliculus  -, 

Trochlear  nerve ». 

Inferior  colliculus- 

Brachium  pontis 

Brachium  conjunctivum'* 

Restiform  body 

Vestibular  n 

Cochlear  n  — -^1 

Dorsal  cochlear  nucleus 

Glossopharyngeal  nerve  ~" 

Vagus  nerve  and  restiform  body'' 

Accessory  nerve — j__^ 

Clava'""  _-  - 

Cuneate  tubercle'"' 


Acoustic  nerve 


A  atcrior  perforated  substance 
/Optic  tract 
'Optic  nerve 
■Infundibulum 
■  Mummillary  body 
,-  Hypophysis 
..^'Oculomotor  nerve 
J        -  Transverse  peduncular  tract 
Cerebral  peduncle 
Pons 

A  bducens  nerve 
f—  Trigeminal  nerve 
-  Facial  nerve 
v  Trapezoid  body 

^  Olive 

*•  Tractus  lateralis  minor 
*~  Hypoglossal  nerve 


Fig.  87. — Lateral  view  of  brain  stem  of  the  sheep. 


forms  one  of  the  nuclei  of  this  nerve  (Fig.  111).  This  bundle  of  fibers  and  the 
associated  mass  of  gray  matter  are  known  as  the  spinal  tract  and  nucleus  of  the 
spinal  tract  of  the  trigeminal  nerve. 

The  restiform  body  (corpus  restiforme  or  inferior  cerebellar  peduncle)  lies 
between  the  lateral  border  of  the  fourth  ventricle  and  the  roots  of  the  vagus 
and  glossopharyngeal  nerves  in  the  rostral  part  of  the  medulla  oblongata  (Figs. 
87-89).  There  is  no  sharp  line  of  demarcation  between  it  and  the  more  cau- 
dally  placed  clava  and  cuneate  tubercle.  It  is  produced  by  a  large  strand  of 
nerve-fibers,  which  run  along  the  lateral  border  of  the  fourth  ventricle  and  then 
turn  dorsally  into  the  cerebellum.     These  fibers  serve  to  connect  the  medulla 


AN  \Im\IY    OF     III!.    PONS 

oblongata  and  spinal  cord  on  the  one  hand  with  the  cerebellum  on  the  other. 
By  a  careful  inspection  of  the  surface  of  the  medulla  it  is  possible  to  recognize 
the  source  of  some  of  the  fibers  entering  into  the  composition  of  the  restiform 
body.  The  ventral  external  arcuate  fibers  can  be  seen  entering  it  after  crossing 
over  the  surface  of  the  lateral  area;  and  the  dorsal  spinocerebellar  trad  can  also 
be  traced  into  it  Erom  a  position  dorsal  to  the  caudal  extremity  of  the  olive. 

At  the  point  where  the  restiform  body  begins  to  turn  dorsally  toward  the 
cerebellum,  it  is  partly  encircled  by  an  elongated  transversely  placed  elevation 
formed  by  the  ventral  and  dorsal  cochlear  nuclei  (Figs.  87,  88).  This  ridge  is 
continuous  on  the  one  hand  with  the  cochlear  nerve,  and  on  the  other  with 
several  bundles  of  fibers  which  run  medialward  over  the  floor  of  the  fourth 
ventricle  .:nd  are  known  as  the  stria  mcdullares  acusticcc  (Fig.  89).  The  co<  blear 
nuclei  are  more  prominent  in  the  sheep,  while  the  medullary  stria-  are  best  seen 
in  the  human  brain.  Just  caudal  to  this  ridge  there  is  sometimes  seen  another, 
running  more  obliquely  across  the  restiform  body,  which  is  an  outlying  portion 
of  the  pons  and  has  been  described  by  Essick  (1907)  under  the  name  corpus 
pontobulbare. 

Nerve  Roots. — From  the  surface  of  the  medulla  oblongata  there  emerge  in 
linear  order  along  the  posterior  lateral  sulcus  a  series  of  root  filaments,  which 
continues  the  line  of  the  dorsal  roots  of  the  spinal  nerves.  These  are  the  root- 
lets of  the  glossopJ/aryiigcal,  vagus  and  accessory  nerves.  But  unlike  the  dorsal 
roots,  which  are  made  up  of  afferent  fibers,  the  spinal  accessory  nerve  contains 
efferent  fibers,  while  the  vagus  and  glossopharyngeal  are  mixed  nerves.  The 
line  of  the  ventral  or  motor  roots  of  the  spinal  nerves  is  continued  in  the  medulla 
oblongata  by  the  root  filaments  of  the  hypoglossal  nerve,  which  is  also  composed 
of  motor  fibers.  The  abducens.  facial,  and  acoustic  nerves  make  their  exit  along 
the  caudal  border  of  the  pons  in  the  order  named  from  within  outward.  The 
abducens  emerges  between  the  pons  and  the  pyramid,  the  acoustic  far  lateral- 
ward  in  line  with  the  restiform  body,  and  the  facial  with  its  sensory  root,  the 
ncrvus  intermedins,  near  the  acoustic  nerve  (Figs.  86-88). 

THE  ANATOMY  OF  THE  PONS 

The  pons,  which  is  differentiated  from  the  ventral  part  of  the  metencephalon, 
is  interposed  between  the  medulla  oblongata  and  the  cerebral  peduncles  and 
lies  ventral  to  the  cerebellum.  As  seen  from  the  ventral  surface,  it  is  formed 
by  a  broad  transverse  band  of  nerve-fibers,  which  on  either  side  become  aggre- 
gated into  a  large  rounded  strand,  the  brae  hi  urn  ponds  or  middle  cerebellar 


124 


THE    NERVOUS    SYSTEM 


peduncle,  and  finally  enter  the  corresponding  hemisphere  of  the  cerebellum 
(Figs.  83,  86).  This  transverse  band  of  fibers,  which  gives  the  bridge-like 
form  from  which  this  part  derives  its  name,  belongs  to  the  basilar  portion  of 
the  pons  and  is  superimposed  upon  a  deeper  dorsal  portion  that  may  be  regarded 
as  a  direct  upward  continuation  of  the  medulla  oblongata.  The  transverse 
fibers  form  a  part  of  the  pathway  connecting  the  cerebral  hemispheres  with  the 
opposite  cerebellar  hemispheres;  and  the  size  of  the  pons,  therefore,  varies  with 


Anterior   limb    of  ^     / 
internal  capsule 


I 
Head  of  the  can-  S 
date  nucleus 


Corona  rod  iota 


--'  Tail  of  the  caudate  nucleus 
Lenticulotka- 

Itimic  part 


..  Rttrolcnlicular  '' 


part 

-  Sublenticular 

part 

-  Thalamus 


J 


'  Posterior 
limb  of 

internal 
capsule 


A nlerior  commissure'' 
Anterior  perforated,- 
substance  y' 

Optic  nerve      ,--' 
Basis  pedunculi'' 

Pons'"'     ,_ 
Ncrvus     j portio  minor'"    _,,■ 
trigeminus  \portio  major''" 

Acoustic  nerve 

Facial  nerve 

Glossopharyngeal  and  vagus  nerves  •*--- ™  -  ~ ' 

Olive Z2IJ 

Hypoglossal  nerve \    \ 

Ventral  external  arcuate  fibers 5 

Pyramid 

Ventral  root  N.  cerv.  I  -=-'-- 

Anterior  lateral  sulcus ''"_,— 

Ventral  root  N.  cerv.  11"''-' 


Medial  geniculate  body 
'--Superior  colliculus 

""Inferior  quadrigeminal  brachium 

"^•Inferior  colliculus 
~~ -Trochlear  nerve 
"--  Lateral  lemniscus 

--  Brachium  conjunctivum 

•  Fila  lateralia  ponds 
-  Dentate  nucleus 
-  Restiform  body 
•  Brachium  pontis 

"'  Dorsal  cochlear  nuc. 
^    "  Corpus  pontobulbar 
"'Restiform  body 
Tuber culum  cinereum 
~~'  'Accessory  nerve 


---Dorsal  root  N.  cerv.  II 


Fig.  88. — Lateral  view  of  human  brain  stem. 

the  size  of  these  other  structures.  It  is  instructive  to  compare  the  brains  of 
the  shark,  sheep,  and  man  with  this  point  in  mind  (Figs.  11,  84,  85). 

The  ventral  surface  of  the  pons  is  convex  from  above  downward  and  from 
side  to  side  and  rests  upon  the  basilar  portion  of  the  occipital  bone  and  upon 
the  dorsum  sellae  (Fig.  81).  A  groove  along  the  median  line,  the  basilar  sulcus, 
lodges  the  basilar  artery  (Fig.  86). 

The  trigeminal  nerve  emerges  from  the  ventral  surface  of  the  pons  far  lateral- 
ward  at  the  point  where  its  constituent  transverse  fibers  are  converging  to  form 


I  III       I  PI  A    R  I  II     \  IN  I  Kit    II, 

the  brachium  pontis.  In  fact,  it  is  customary  to  take  the  exit  of  this  nerve  as 
marking  the  point  of  junction  of  the  pons  with  its  brachium.  The  nen  <•  has  two 
roots  which  lie  close  together:  the  larger  is  the  sensory  root,  or  portio  major; 
the  smaller  is  the  motor  root,  or  portio  minor  (Fig.  88). 

The  posterior  surface  of  the  pons  forms  the  rostral  part  of  the  floor  of  the 
fourth  ventricle,  along  the  lateral  borders  of  which  there  are  two  prominent 
and  rather  large  strands  of  nerve  fibers,  the  brachia  conjunctiva  (Figs.  88,  vn 

The  brachia  conjunctiva  or  superior  cerebellar  peduncles  lie  under  cover  of 
the  cerebellum.  As  they  emerge  from  the  white  center^  of  the  cerebellar  hemi- 
spheres they  curve  rostrally  and  take  up  a  position  along  the  lateral  border  of 
the  fourth  ventricle.  They  converge  as  they  ascend  and  disappear  from  view 
by  sinking  into  the  substance  of  the  mesencephalon  under  cover  of  the  inferior 
quadrigeminal  bodies.  Each  consists  of  fibers  which  connect  the  cerebellum 
with  the  red  nucleus,  a  large  gray  mass  situated  within  the  midbrain  ventral  to 
the  superior  colliculus  of  the  corpora  quadrigemina.  The  interval  between  the 
two  brachia  conjunctiva,  where  these  form  the  lateral  boundaries  of  the  fourth 
ventricle,  is  occupied  by  a  thin  lamina  of  white  matter,  the  anterior  medullary 
velum  (Fig.  85).  This  is  stretched  between  the  free  dorsomedial  borders  of  the 
two  brachia  and  forms  the  roof  of  the  rostral  portion  of  the  ventricle.  Cauda  11  v 
it  is  continuous  with  the  white  center  of  the  cerebellum.  The  fibers  of  the 
trochlear  nerves  decussate  in  the  anterior  medullary  velum  and  emerge  from  its 
dorsal  surface  (Fig.  89).  As  they  run  through  the  velum  the}'  produce  a  raised 
white  line  which  extends  transversely  from  one  brachium  to  the  other. 

THE  FOURTH  VENTRICLE 

The  lozenge-shaped  cavity  of  the  rhombencephalon  is  known  as  the  fourth 
ventricle.  It  lies  between  the  pons  and  medulla  oblongata,  ventrally,  and  the 
cerebellum  dorsally,  and  is  continuous  with  the  central  canal  of  the  closed  por- 
tion of  the  medulla,  on  the  one  hand,  and  with  the  cerebral  aqueduct  on  the 
other  (Fig.  84).  On  each  side  a  narrow  curved  prolongation  of  the  cavity  ex- 
tends laterally  on  the  dorsal  surface  of  the  restiform  body.  This  is  known  as 
the  lateral  recess  (Figs.  89.  90).  It  opens  into  the  subarachnoid  space  near  the 
flocculus  of  the  cerebellum;  and  through  this  lateral  aperture  of  the  fourth  ven- 
tricle (foramen  of  Luschka)  protrudes  a  small  portion  of  the  chorioid  plexus 
(Fig.  90).  There  is  also  a  median  aperture  (foramen  of  Magendie)  through  the 
roof  of  the  ventricle  near  the  caudal  extremity.  By  means  of  these  three  open- 
ings, one  medial  and  two  lateral,  the  cavity  of  the  ventricle  is  in  communica- 


126 


THE    NERVOUS    SYSTEM 


tion  with  the  subarachnoid  space,  and  cerebrospinal  fluid  may  escape  from  the 
former  into  the  latter. 

The  floor  of  the  fourth  ventricle  is  known  as  the  rhomboid  fossa  and  is  formed 
by  the  dorsal  surfaces  of  the  pons  and  open  part  of  the  medulla  oblongata,  which 
are  continuous  with  each  other  without  any  line  of  demarcation  and  are  irreg- 
ularly concave  from  side  to  side  (Figs.  89,  91).  The  fossa  is  widest  opposite  the 
points  where  the  restiform  bodies  turn  dorsally  into  the  cerebellum;  and  it 
gradually  narrows  toward  its  rostral  and  caudal  angles.    The  lateral  boundaries 


Thalamus 


Medial  geniculate  bod\ 

Inferior  quadrigeminal  __---     ~"\ 
brachium 

Frenulum  veli 

Anterior  medullary  velum 

Brae liium  eonjunetivum  — 

Braeliiu m  pontis~-~~-  - 
Restiform  body~-._ 

Superior  fovea \T' 

Area  acustica-=~'^. 

Inferior  fovea 

Restiform  body —  - 

Ala  cinerea--- 

Funieulus  scparans—'' 

Area  postrema-'' 

Obex-''' 

Funiculus  gracilis-'' 

Funiculus  cuneatus--' 


Pineal  body 


-  Superior  colliculus 

Inferior  colliculus 

—  Cerebral  peduncle 

Trochlear  nerve 

Median  sulcus 

Locus  carulcus 

-  -  Facial  colliculus 

i Medial  eminence 

Sulcus  limilans 

Lateral  recess 

Stricr  mcdullares 
Teen  i 'a 

Trigonum  hypoglossi 
~"Cuneatc  tubercle 
~"~~Tubcrculum  cinereum 
""-Clava 

'--Posterior  median  fissure 
~ ^Posterior  intermediate 
sulcus 
^Posterior  lateral  sulcus 


Fig.  89. — Dorsal  view  of  human  brain  stem. 


of  the  fossa,  which  are  raised  some  distance  above  the  level  of  the  floor,  are 
formed  by  the  following  structures:  the  brachia  conjunctiva,  restiform  bodies, 
cuncatc  tubercles,  and  clavce.  Of  the  four  angles  to  the  rhomboid  fossa,  two 
are  laterally  placed  and  correspond  to  the  lateral  recesses.  At  its  caudal  angle 
the  ventricle  is  continuous  with  the  central  canal  of  the  closed  part  of  the  me- 
dulla oblongata,  and  at  its  rostral  angle  with  the  cerebral  aqueduct.  Joining 
the  two  last  named  angles  there  is  a  median  sulcus  which  divides  the  fossa  into 
two  symmetric  lateral  halves. 

The  rhomboid  fossa  is  arbitrarily  divided  into  three  parts.     The  superior 


I  ill      FOURTH    \  l  \  l  RIl  I  i; 

part  is  triangular,  with  its  apex  directed  rostrally  and  its  base  along  an  im 
an-  line  through  the  superior  foveae.  The  inferior  part  is  also  triangular,  bul 
with  its  apex  directed  caudally  and  its  base  at  the  level  of  the  horizontal  por- 
tions of  the  taeniae  of  the  ventricle.  Between  these  two  triangular  portions  is 
the  intermediate  part  of  the  fossa,  which  is  prolonged  outward  into  the  lateral 
recesses.  The  Boor  is  covered  with  a  thin  lamina  of  gray  matter  continuous 
with  that  which  lines  the  central  canal  and  cerebral  aqueduct.  Crossing  the 
fossa  transversely  in  its  intermediate  portion  are  several  strands  of  fibers  known 
as  the  stria  meduUares  acustica.  These  arc  subject  to  considerable  variation  in 
different  specimens.  Springing  from  the  dorsal  cochlear  nuclei  they  wind 
around  the  restiform  body  in  the  lateral  recess  and  run  transversely  across  the 
fossa  to  disappear  in  the  median  sulcus. 

The  inferior  portion  of  the  fossa  bears  some  resemblance  to  the  point  of  a 
pen  and  has  been  called  the  calamus  scriptorius.  It  belongs  to  the  medulla 
oblongata.  In  this  part  of  the  fossa  there  is  on  either  side  a  small  depression, 
the  inferior  fovea,  shaped  like  an  arrow-head,  the  point  of  which  is  directed  toward 
the  stria'  medullares.  From  the  basal  angles  of  this  triangle  run  diverging  sulci: 
a  medial  groove  toward  the  opening  of  the  central  canal  and  a  lateral  groove 
more  nearly  parallel  to  the  median  sulcus.  By  these  sulci  the  inferior  portion 
of  the  fossa  is  divided  into  three  triangular  areas.  Of  these  the  most  medial 
is  called  the  trigone  of  the  hypoglossal  nerve  or  trigonum  nervi  liypoglossi.  Be- 
neath the  medial  part  of  this  slightly  elevated  area  is  located  the  nucleus  of  the 
hypoglossal  nerve.  The  area  betw-een  the  two  sulci,  which  diverge  from  the 
fovea  inferior,  is  the  ala  cinerea  or  triangle  of  the  vagus  nerve.  Both  names 
are  appropriate,  the  one,  because  of  its  gray  color,  and  the  other,  because  a 
nucleus  of  the  vagus  nerve  lies  subjacent  to  it.  The  third  triangular  field, 
placed  more  laterally,  forms  a  part  of  the  area  acustica. 

The  area  acustica  is,  however,  not  restricted  to  the  inferior  portion  of  the 
fossa,  but  extends  into  the  intermediate  part  as  well.  Here  it  form-  a  prominent 
elevation  over  which  the  stria?  medullares  run.  Subjacent  to  this  area  lie  the 
nuclei  of  the  vestibular  nerve.  A  part  of  the  acoustic  area  and  all  of  the  ven- 
tricular floor  rostral  to  it  belong  to  the  pons. 

Rostral  to  the  stria?  medullares  there  may  be  seen  a  shallow  depression, 
the  fovea  superior,  medial  to  which  there  is  a  rounded  elevation,  the  facial 
colliciilus.  Under  cover  of  this  eminence  the  fibers  of  the  facial  nerve  bend 
around  the  abducens  nucleus.  Extending  from  the  fovea  superior  to  the 
cerebral  aqueduct  is  a  shallow  groove,  usually  faint   blue  in  color,  the  locus 


121 


THE    NERVOUS    SYSTEM 


car  ulcus,  beneath  which  lies  the  substantia  ferruginea,  composed  of  pigmented 
nerve-cells. 

Beginning  at  the  cerebral  aqueduct  and  extending  through  both  the  superior 
and  inferior  fovea?  is  a  very  important  groove,  the  sulcus  limitans,  which  repre- 
sents the  line  of  separation  between  the  parts  derived  from  the  alar  plate  and 
those  which  originate  from  the  basal  plate  of  the  embryonic  rhombencephalon. 
Lateral  to  this  sulcus  lie  the  sensory  areas  of  the  ventricular  floor,  including  the 
area  acustica,  all  of  which  are  derived  from  the  alar  plate.  Medial  to  this 
sulcus  there  is  a  prominent  longitudinal  elevation,  known  as  the  medial  eminence, 
which  includes  two  structures  already  described,  namely,  the  facial  colliculus 
and  the  trigone  of  the  hypoglossal  nerve.     Beneath  the  medial  part  of  this 


Tela  chorioidea 

Chorioid  plexus 

Median  aperture  of 
fourth  ventricle 


Fig.  90. — Dorsal  view  of  human  rhombencephalon  showing  tela  chorioidea  and  chorioid  plexus  of 

the  fourth  ventricle. 


trigone  lies  the  nucleus  of  the  hypoglossal  nerve  and  beneath  the  lateral  part  is  a 
group  of  cells  designated  as  the  nucleus  inter calatus. 

One  or  two  features  remain  to  be  mentioned.  'At  the  caudal  end  of  the  ala 
cinerea  is  a  narrow  translucent  obliquely  placed  ridge  of  thickened  ependyma, 
known  as  the  funiculus  scparans.  Between  this  ridge  and  the  clava  is  a  small 
strip  of  the  ventricular  floor,  called  the  area  post  rem  a.  which  on  microscopic 
examination  is  found  to  be  rich  in  blood-vessels  and  neurogliar  tissue. 

The  roof  of  the  fourth  ventricle  is  formed  by  the  anterior  medullary  velum, 
a  small  part  of  the  white  substance  of  the  cerebellum,  and  by  the  tela  chorioidea 
lined  internally  by  cpendymal  epithelium  (Fig.  85).  Caudal  to  the  cerebellum 
the  true  roof  of  the  cavity  is  very  thin  and  consists  only  of  a  layer  of  ependymal 
epithelium,  which  is  continuous  with  that  lining  the  other  walls  of  the  ventricle. 


w  \i"M\    OP    mi.    mi  -i  \<  i  I'll  \i 

This  is  supported  on  its  outer  surface  by  a  layer  of  pia  mater,  the  tela  chorio 
rich  in  blood  vessels.     From  this  layer  vascular  tufts,  covered  by  epithelium, 
are  invaginated  Into  the  cavity  and  form  the  chorioid  plexus  of  the  fourth  veil 
trick-  (Fig.  90).    The  plexus  is  invaginated  along  two  vertical  lines  close  to  the 

median  plane  and  along  two  horizontal  lines,  which  diverge  at  right  angles  from 
the  vertical  ones  and  run  toward  the  lateral  recesses.  These  righl  and  left 
halves  are  joined  together  at  the  angles  so  that  the  entire  plexus  has  the  shape 

of  the  letter  T.  the  vertical  limb  of  which .  however,  is  double. 

After  the  tela  chorioidea  with  its  epithelial  lining  has  been  torn  away  to 
expose  the  lloor  of  the  ventricle,  there  remains  attached  to  the  lateral  bound- 
aries of  the  caudal  part  of  the  cavity  the  torn  edges  of  this  portion  of  the  roof. 
These  appear  a-  line-,  the  teenies  of  the  fourth  ventricle,  which  meet  over  the 
caudal  angle  of  the  cavity  in  a  thin  triangular  lamina,  the  obex  (Fig.  89).  Ros- 
t rally  each  taenia  turns  lateralward  over  the  restiform  body  and  forms  the  caudal 
boundary  of  the  corresponding  lateral  recess. 

THE  MESENCEPHALON 

The  midbrain  or  mesencephalon  occupies  the  notch  in  the  tentorium  and 
connects  the  rhombencephalon,  on  the  one  side  of  that  shelf-like  process  of 
dura,  with  the  prosencephalon  on  the  other  (Fig.  81).  It  consists  of  a  dorsal 
part,  the  corpora  quadrigemina.  and  a  larger  ventral  portion,  the  cerebral  pe- 
duncles. It  is  tunneled  by  a  canal  of  relatively  small  caliber,  called  the  cerebral 
aqueduct,  which  connects  the  third  and  fourth  ventricles  and  is  placed  nearer 
the  dorsal  than  the  ventral  aspect  of  the  midbrain  (Fig.  84). 

The  cerebral  peduncles  (pedunculi  cerebri,  crura  cerebri),  as  seen  on  the 
ventral  aspect  of  the  brain,  diverge  like  a  pair  of  legs  from  the  rostral  border  of 
the  pons  (Fig.  83).  Just  before  they  disappear  from  view  by  entering  the  ven- 
tral surface  of  the  prosencephalon  they  enclose  between  them  parts  of  the  hypo- 
thalamus, and  are  encircled  by  the  optic  tracts.  On  section,  each  peduncle  i- 
seen  to  be  composed  of  a  dorsal  part,  the  tegmentum,  and  a  ventral  part,  the 
basis  pedunculi.  Between  the  basis  pedunculi  and  the  tegmentum  there  inter- 
venes a  strip  of  darker  color,  the  substantia  nigra  (Fig.  113).  By  dissection  it  i> 
easy  to  show  that  the  basis  pedunculi  is  composed  of  longitudinally  coursing 
fibers  which  can  be  traced  rostrally  to  the  internal  capsule  (Fig.  88).  In  the 
other  direction  some  of  these  fibers  can  be  followed  into  the  corresponding  pyra- 
mid of  the  medulla  oblongata.  On  the  surface  two  longitudinal  sulci  mark  the 
plane  of  separation  between  the  tegmentum  and  the  basis  pedunculi.     The 


13° 


THE   NERVOUS    SYSTEM 


groove  on  the  medial  aspect  of  the  peduncle,  through  which  emerge  the  fibers 
of  the  third  nerve,  is  known  as  the  sulcus  of  the  oculomotor  nerve,  while  that  on 
the  lateral  aspect  is  called  the  lateral  sulcus  of  the  mesencephalon.  Dorsal  to 
this  latter  groove  the  tegmentum  comes  to  the  surface  and  is  faintly  marked  by 
line  bundles  of  fibers  which  curve  dorsally  toward  the  inferior  colliculus  of  the 
corpora  quadrigemina  (Fig.  88).  These  fibers  belong  to  the  lateral  lemniscus, 
the  central  tract  associated  with  the  cochlear  nerve. 

The  corpora  quadrigemina  form  the  dorsal  portion  of  the  mesencephalon, 
and  consist  of  four  rounded  eminences,  the  quadrigeminal  bodies  or  colliculi, 


Anterior  limb  of  internal  capsule.^ 
Stria  terminalis 
Ilalu  jiular  commissurt  y 
Habenular  trigone,  } 

Pineal  body 
Posterior  limb  of  internal  capsule. 

Superior  colliculus     ^S? 
Optic  radiation 

Attachment  anterior  ' 
medullary  velum 
Inferior  colliculus 

Superior  fovea    v 
Brachium  conjunetivum  ^s. 

Brachium  pontis 

Restiform  body 

Dorsal  cochlear  nucleus 

Acoustic  area 

Inferior  fovea  and  restiform  body  -■'- 

Tania  of  fourth  ventricle- 
Clava 
Cunealc  tubercle 
Posterior  lateral  sulcus 


—  Corona  radiata 
-Head  of  caudate  nucleus 
Stria  medullaris  of  thalamus 

,,  Third  ventricle 

,  Thalamus 

,,  Tail  of  caudate  nucleus 

.  ■■  -Pulvinar 
\,  Median  sulcus 

y)S  K  Trochlear  nerve 

''  /Facial  colliculus 
■'„''  Trigeminal  nerve 

Sulcus  limilaus 
^-Medial  eminence 
,  A/a  cinerea 

-  Lateral  recess  of  fourth  ventricle 
Trigone  of  hypoglossal  nerve 
Obex 
/-■Posterior  median  fissure 
Posterior  intermediate  sulcus 
-Funiculus  gracilis 
-  Funiculus  cuneatus 


Fig.  91. — Dorsal  view  of  brain  stem  of  sheep. 


which  arise  from  the  dorsal  aspect  of  a  plate  of  mingled  gray  and  white  matter 
known  as  the  quadrigeminal  lamina  (Figs.  89,  91).  The  superior  colliculi  are 
larger  than  the  inferior,  the  disproportion  being  greater  in  the  sheep  than  in 
man.  A  median  longitudinal  groove  separates  the  colliculi  on  either  side.  In 
the  rostral  end  of  this  groove  rests  the  pineal  body,  while  attached  to  its  caudal 
end  is  a  band  which  runs  to  the  anterior  medullary  velum,  and  is  known  as  the 
frenulum  veli.  A  transverse  groove  runs  between  the  superior  and  inferior  collic- 
uli and  extends  on  to  the  lateral  aspect  of  the  mesencephalon,  where  it  inter- 
venes between  the  superior  colliculus  and  the  inferior  quadrigeminal  brachium 
(Figs.  87,  89). 


A\  \  i<  i\l\    OP    I  Ml     Mi  SI  v  I  I'll  \l.<>\ 


The  Brachia  of  the  Corpora  Quadrigemina.  From  each  coUiculus  there  runs 
ventrally  and  rostrally  on  the  lateral  aspeel  of  the  mesencephalon  an  arm  or 
brachium  ili.ur-.  87,  88).  The  interior  quadrigeminal  brachium  is  the  more  con 
SpiCUOUS  and  is  the  only  one  that  can  be  readily  identified  in  the  sheep.  It 
runs  from  the  interior  colliculus  to  the  medial  geniculate  body.  This  ban  oval 
eminence,  belonging  to  the  diencephalon,  which  has  been  displaced  caudally  so 
as  to  lie  on  the  lateral  aspeel  of  the  mesencephalon.  The  superior  quadrigeminal 
brachium  runs  from  the  superior  colliculus  toward  the  lateral  geniculate  body, 
passing  between  the  pulvinar  of  the  thalamus  and  the  medial  geniculate  body. 
Some  of  the  fibers  can  be  traced  beyond  the  lateral  geniculate  body  into  the 
Optic  tract. 


CHAPTER  IX 

THE  STRUCTURE  OF  THE  MEDULLA  OBLONGATA 

The  medulla  oblongata  contains  the  nerve-cells  and  fiber  tracts  associated 
with  certain  of  the  cranial  nerves.  These  include  the  central  mechanisms  which 
control  the  reflex  activities  of  the  tongue,  pharynx,  and  larynx,  and  in  part  those 
of  the  thoracic  and  abdominal  viscera  also.  At  the  same  time  the  ascending 
and  descending  fiber  tracts,  which  unite  the  spinal  cord  with  higher  nerve 
centers,  pass  through  the  medulla  oblongata. 

The  central  connections  of  the  cranial  nerves,  except  those  of  the  first  two 
pairs,  are  located  in  the  medulla  oblongata  and  in  the  tegmental  portions  of  the 
pons  and  mesencephalon.  In  many  respects  they  resemble  the  connections  of 
the  spinal  nerves  within  the  spinal  cord.  The  following  general  statements  on 
this  topic,  most  of  which  are  illustrated  in  Fig.  92,  will  help  to  elucidate  the 
structure  of  the  brain  stem. 

1.  The  cells  of  origin  of  the  sensory  fibers  of  the  cranial  nerves  (Fig.  92,  1) 
are  found  in  ganglia  which  lie  outside  the  cerebrospinal  axis  and  are  homologous 
with  the  spinal  ganglia.  These  are  the  semilunar  ganglion  of  the  trigeminal, 
the  geniculate  ganglion  of  the  facial,  the  superior  and  petrous  ganglia  of  the 
glossopharyngeal,  the  jugular  and  nodose  ganglia  of  the  vagus,  the  spiral  gang- 
lion of  the  cochlear,  and  the  vestibular  ganglion  of  the  vestibular  nerve. 

2.  All  of  these  sensory  ganglia  except  the  last  two,  the  cells  of  which  are 
bipolar,  are  formed  by  unipolar  cells,  the  axons  of  which  divide  dichotomously 
into  peripheral  and  central  branches.  The  latter  (or  in  the  case  of  the  acoustic 
nerve  the  central  processes  of  the  bipolar  cells)  form  the  sensory  nerve  roots, 
enter  the  brain  stem  and  divide,  each  into  a  short  ascending  and  a  long  descending 
branch.  These  branches  give  off  numerous  collaterals,  which  with  the  terminal 
branches  end  in  gray  masses  known  as  sensory  nuclei  or  nuclei  of  termination. 
It  is  the  descending  branches  of  the  sensory  fibers  of  the  trigeminal  nerve  which 
form  the  spinal  tract  of  that  nerve  illustrated  in  Figs.  92,  98,  99,  101. 

3.  The  ascending  branch  may  be  entirely  wanting,  as  in  the  case  of  the  sen- 
sory fibers  of  the  seventh,  ninth,  and  tenth  nerves,  all  of  which  bend  caudally  and 
form  a  descending  tract  in  the  medulla  oblongata,  known  as  the  tractus  soli- 
tarius  (Figs.  92,  101,  103). 

132 


[HE    STRUCTURE    01    nil     Ml  DULLA    0BL0NGA1  \ 


4.  The  sensory  nuclei  (Fig.  92,  4),  within  which  the  afferenl  fibers  terminate, 
contain  the  cells  of  origin  of  the  sensory  fibers  oj  the  second  order    I 

Some  of  these  are  short;  others  are  long,  and  these  may  be  cither  direci  01 
crossed.    Many  of  them  divide  Into  ascending  and  descending  branches.    They 

run  in  the  reticular  formation  and  some  <)!'  the  ascending  fibers  reach  the  thal- 
amus. 

5.  These  sensory  fibers  of  the  second  order  give  off  collaterals  to  the  motor 
nuclei.  Direct  collaterals  from  the  sensory  fibers  of  the  cranial  nerves  to  the 
motor  nuclei  are  few  in  number  or  entirely  wanting. 

6.  The  motor  nuclei  (Fig.  92,  5)  are  aggregations  of  multipolar  cells  which 
give  origin  to  the  motor  fibers  of  the  cranial  nerves  (Fig.  92,  3). 


-  Main  sensory  nucleus 
of  trigeminal  nerve 

>ii  fiber  of 
ond  order 


Tractus  solitarius 
Nucleus  of 

hypoglossal  nerve 
Afferent  fiber  of 

second  order 
Spinal  Irarl  of 

trigeminal  nenc 

and  ils  nucleus 


Fig.  92. — Diagram  of  the  tongue  and  rhombencephalon  to  illustrate  the  central  connections 
and  functional  relationships  of  certain  of  the  cranial  nerves:  1,  Sensory  neurons  of  the  first  order 
of  the  trigeminal  and  glossopharyngeal  nerves;  2,  sensory  neurons  of  the  second  order;  3,  motor 
fibers  of  the  hypoglossal  nerve;  4,  sensory  nuclei;  5,  motor  nucleus  of  hypoglossal  nerve.      (Cajal.) 


The  Rearrangement  Within  the  Medulla  Oblongata  of  the  Structures  Con- 
tinued Upward  from  the  Spinal  Cord.— At  the  level  of  the  rostral  border  of  the 
first  cervical  nerve  the  spinal  cord  goes  over  without  a  sharp  line  of  demarcation 
into  the  medulla  oblongata.  The  transition  is  gradual  both  as  to  external  form 
and  internal  structure;  but  in  the  caudal  part  of  the  medulla  there  occurs  a 
gradual  rearrangement  of  the  fiber  tracts  and  alterations  in  the  shape  of  the 
gray  matter,  until  at  the  level  of  the  olive,  a  section  of  the  medulla  bears  no 
resemblance  to  one  through  the  spinal  cord. 

The  realignment  of  the  corticospinal  tracts  and  the  termination  of  the  fibers 
of  the  posterior  funiculi  of  the  spinal  cord  are  two  of  the  most  important  factors 


134  THE    NERVOUS    SYSTEM 

responsible  for  this  gradual  transformation.  Traced  rostrally  from  the  spinal 
cord,  the  ventral  corticospinal  tracts  are  seen  to  enter  the  pyramids  within  the 
ventral  area  of  the  medulla  oblongata,  that  is  to  say,  they  enter  the  medulla 
without  realignment.  But  the  fibers  of  the  lateral  corticospinal  tracts  on  enter- 
ing the  medulla  swing  ventromedially  in  coarse  bundles,  which  run  through 
the  anterior  gray  columns  and  cut  them  off  from  the  gray  matter  surrounding 
the  central  canal  (Figs.  93,  95).  After  crossing  the  median  plane  in  the  decussa- 
tion of  the  pyramids  these  fibers  join  those  of  the  opposite  ventral  corticospinal 
tracts  and  form  the  pyramids  (Fig.  96).  Thus  fibers  from  the  lateral  funiculus 
come  to  lie  ventral  to  the  central  canal  and  displace  this  dorsally;  and  at  the  same 
time  a  start  is  made  toward  breaking  up  the  H -shaped  gray  figure  characteristic 
of  the  spinal  cord. 

Cerebral  hemisphere 


ylkvP'M      Spinal 
VSs^yjy        cord 

Fig.  93. — Diagram  of  the  corticospinal  tracts. 

Shortly  after  entering  the  medulla  oblongata  the  fibers  of  the  posterior  funiculi 
end  in  nuclear  masses  which  invade  the  funiculus  gracilis  and  funiculus  cuneatus 
as  expansions  from  the  posterior  gray  columns  and  central  mass  of  gray  sub- 
stance (Figs.  95,  96).  These  are  known  as  the  nucleus  gracilis  and  nucleus  cu- 
neatus. They  cause  a  considerable  increase  in  the  size  of  the  posterior  funiculi 
and  a  corresponding  ventrolateral  displacement  of  the  posterior  columns  of 
gray  matter.  The  fibers  of  the  posterior  funiculi  end  in  these  nuclei  about  cells, 
the  axons  of  which  run  ventromedially  as  the  axis-cylinders  of  internal  arcuate 
fibers.  These  sweep  in  broad  curves  through  the  gray  substance,  and  decus- 
sate ventral  to  the  central  canal  in  what  is  known  as  the  decussation  of  the  medial 
lemniscus.     After  crossing  the  median  plane  they  turn  rostrally  between  the 


TI1K    STRUCT!  RE    OF    llli:    Ml  in  i.i.\    OBLONGA1  \ 


pyramids  and  the  central  gray  matter  to  form  on  either  side  of  the  median 

plain-  a    broad    hand   of   fibers   known    as   the  medial  leninism       I  ;        96    ''7 


Fascu  ii/hs  gracilis - 
I  ast  iculus  cuneatus 
Dorsolateral  fascA  Lissauer)  ^ 
Substantia  gelatinosa 
Dorsal  column  " 
Lateral  corticospinal  tract  " 
(  '<  ntral  canal 
Ventral  column 
Ventral  corticospinal  trad 


■  Funu  iiln  .  grot  ilis 

. A  •.■ilis 

—  1- uni<  iiln-.  <  uneatus 

Spinal  trad  of tl 

1  spinal  Ira 
I '     al  column  \    V 

Lateral  <  orlico  pinal  trait 
-  ( '<  ntral  canal 
~/~"  Dei  USSation  of  the  pyramids 
Vi  ntral  i  olunin 


Fie,  94. 


Fig.  95. 


Funiculus  gracilis 

Nucleus  gracilis 

Funiculus  <  uneatus 

Nucleus  cuneatus 

Spinal  tract  of  trigeminal  n 

Nucleus  of  spinal  tract  of  N.V 

Central  way  matter 

Internal  arcuate  fibers 

Central  canal 

R<  tit  ular  substam  e 

Medial  lemniscus 

Decussation  of  medial  lemniscus 

Decussation  of  the  pyramids 

Pyramid,  corticospinal  tract 


Fie.  96. 


-■  Fourth  ventricle 
..-Dorsal  motor  nucleus  of  vagus 
-■  Nucleus  of  hypoglossal  nerve 
Tractus  solitarius 
Nucleus  of  spinal  tract  of  N.  V 
Spinal  tract  of  trigeminal  nerve 
Fibers  of  hypoglossal  >:■ 
Reticular  substance 
Dorsal  accessory  olivary  nucleus 
Media!  lemniscus 
Inferior  olivary  nucleus 
Medial  accessory  olivary  nucleus 
—  Pyramid,  corticospinal  trad 

Fig.  97. 
Figs.   94-97. — Diagrammatic  cross-sections  to  show   the  relation  of  the  structures  in   the 
medulla   oblongata   to  those   in   the   spinal  cord:   Fig.  94,  First  cervical  segment  of  spinal  cord; 
Fig.  95,  medulla   oblongata,  level   of  decussation  of  pyramids;  Fig.  96,  medulla  oblongata,  level 
of  decussation  of  medial  lemniscus;  Fig.  97,  medulla  oblongata,  level  of  olive. 

At  the  level  of  the  middle  of  the  olive  most  of  the  fibers  of  the  funiculus  cune- 
atus and  funiculus  gracilis  have  terminated  in  their  respective  nuclei;  and  the 
nuclei  also  disappear  a  short  distance  farther  rostrally  (Fig.  97).     With  the 


136  THE    NERVOUS    SYSTEM 

disappearance  of  these  fibers  and  nuclei  there  ceases  to  be  any  nervous  sub- 
stance dorsal  to  the  central  canal,  and  this,  which  has  been  displaced  dorsally 
by  the  accumulation  of  the  corticospinal  fibers  and  those  of  the  lemniscus  ven- 
tral to  it,  opens  out  as  the  floor  of  the  fourth  ventricle  (Fig.  97). 

The  outline  of  the  gray  matter  in  the  most  caudal  portions  of  the  medulla 
oblongata  closely  resembles  that  of  the  spinal  cord.  The  anterior  columns  are 
first  cut  off  by  the  decussation  of  the  pyramids  (Fig.  95).  Then  the  posterior 
columns  are  displaced  ventrolaterally  due  to  the  increased  size  of  the  posterior 
funiculi  and  the  disappearance  of  the  lateral  corticospinal  tracts  from  their 
ventral  aspects.  This  rotation  of  the  posterior  column  causes  the  apex  of 
that  column  with  its  spinal  tract  and  nucleus  of  the  trigeminal  nerve,  which  are 
continuous  with  the  fasciculus  dorsolateral  and  substantia  gelatinosa  of  the 
spinal  cord  (Fig.  94),  to  lie  almost  directly  lateralward  from  the  central  canal 
(Fig.  96).  The  shape  of  the  gray  figure  is  still  further  altered  by  the  develop- 
ment of  special  nuclear  masses,  many  of  which  are  very  conspicuous.  These 
include  the  nucleus  gracilis,  nucleus  cuneatus,  inferior  olivary  nucleus,  and  the 
nuclei  of  the  cranial  nerves.  The  greater  part  of  the  gray  substance  now  becomes 
broken  up  by  nerve-fibers  crossing  in  every  direction,  but  especially  by  the 
internal  arcuate  fibers.  This  mixture  of  gray  and  white  matter  is  known  as  the 
reticular  substance.  The  central  gray  matter  is  pushed  dorsad  first  by  the  pyra- 
mids and  later  by  the  medial  lemniscus  until  it  finally  spreads  out  to  form  a  thin 
gray  covering  for  the  floor  of  the  fourth  ventricle. 

The  Pyramids  and  Their  Decussation. — We  have  had  occasion  repeatedly 
to  refer  to  the  crossing  of  the  lateral  corticospinal  tracts  in  this  and  preceding 
chapters,  but  there  remain  some  details  to  be  presented.  The  pyramids  are 
large,  somewhat  rounded  fascicles  of  longitudinal  fibers,  which  lie  on  either  side 
of  the  anterior  median  fissure  01  the  medulla  oblongata  (Fig.  86).  The  constit- 
uent fibers  take  origin  from  the  large  pyramidal  cells  of  the  anterior  central 
gyrus  or  motor  cerebral  cortex.  The  decussation  of  the  pyramids  or  motor 
decussation  occurs  near  the  caudal  extremity  of  the  medulla  oblongata  (Fig. 
93).  Approximately  the  medial  three-fourths  of  the  corticospinal  tract  passes 
through  the  decussation  into  the  lateral  funiculus  of  the  opposite  side  of  the 
spinal  cord,  as  the  lateral  corticospinal  tract  (fasciculus  cerebrospinalis  lateralis 
or  lateral  pyramidal  tract);  while  the  lateral  one-fourth  is  continued  without 
crossing  into  the  ventral  funiculus  of  the  same  side  as  the  ventral  corticospinal 
tract  (fasciculus  cerebrospinalis  anterior  or  anterior  pyramidal  tract — Figs. 
94,  95,  96,  98).     The  decussating  fibers  are  grouped  into  relatively  large  bundles 


THE    STRU<   ll  KK    <  >F    Till      MIDI   l.l.\    OBLONGATA 


as  tin  s  the  median  plane,  the  bundles  from  one  side  alternating  with 

similar  bundles  from  the  other,  and  largely  obliterating  the  anterior  median  fis- 
sure at  this  level.  There  is  great  individual  variation  as  to  the  relative  size  of 
the  ventral  and  lateral  corticospinal  tracts;  and  there  may  even  be  marked 
asymmetry  due  to  a  difference  in  the  proportion  of  the  decussating  fibers  on  the 
two  sides. 

The  nucleus  gracilis  and   nucleus  cuneatus   (nucleus  funiculi  ,Lrra<  ili-  and 
nucleus  funiculi  cuneati)  are  large  masses  of  gray  matter  located  in  the  pos- 
terior funiculi  of  the  caudal  portion  of  the  medulla  oblongata.     They  arc  sur 
rounded  by  the  libers  of  these  funiculi  except  on  their  ventral  aspects,  where  they 
are  continuous  with  the  remainder  of  the  gray  substance  (Fig.  99).     The  libers 


Funiculus  gracilis- 
Nucleus  gracilis' 


of  trigeminal j^'^V      ^HSift^' '-''■'  i    "?•'"' 'V'.' 


Spinal  tract 
nerve 

Nucleus  of  spinal  tract  of — l3v?\-~V-  v  '£>*   ' 


Central  canal 
Decussation  of  the  pyramids 

Anterior  column 


5&2 


Posterior  median  fissure 
Funiculus  cuneatus 
Nucleus  cuneatus 

Dorsal  spinocerebellar  tract 
Ventral  spinocerebellar  tract 
Ventral  fasciculus  proprius 
Bulbospinal  tract 
Anterior  median  fissure 


Fig.  98. — Section  through  the  medulla  oblongata  of  a  child  at  the  level  of  the  decussation  of  the 
pyramids.     Pal-Weigert  method.      (XO.) 


of  the  gracile  and  cuneate  fasciculi  terminate  in  the  corresponding  nuclei;  and 
their  terminal  arborizations  are  synaptically  related  to  the  neurons,  whose  cell 
bodies  and  dendrites  are  located  there  (Fig.  100).  Accordingly,  in  sections 
through  successive  levels  we  see  the  fibers  decreasing  in  number  as  the  nuclei 
grow  larger  (Figs.  98.  99,  101).  It  is  due  to  the  presence  of  these  nuclei  that  the 
funiculi  become  swollen  to  form  the  club-shaped  prominences  with  which  we  are 
already  familiar  under  the  names  clava  and  cuneate  tubercle.  At  the  level  of  the 
pyramidal  decussation  the  gracile  nucleus  has  the  form  of  a  rather  thin  and 
ill-defined  plate,  while  the  cuneate  nucleus  is  represented  by  a  slight  projection 
from  the  dorsal  surface  of  the  posterior  gray  column  (Fig.  98).  At  the  level  of 
the  decussation  of  the  lemniscus  both  have  enlarged  and  the  gracile  nucleus  has 
become  sharply  outlined  (Fig.  99).     As  the  central  canal  opens  out  into  the 


138 


THE    NERVOUS    SYSTEM 


fourth  ventricle  the  nuclei  are  displaced  laterally  and  gradually  come  to  an  end 
as  the  restiform  body  becomes  clearly  defined  (Fig.  101). 

As  one  would  expect  from  the  fact  that  there  is  no  sharp  line  of  separation  between  the 
spinal  cord  and  medulla  oblongata,  some  of  the  fibers  of  the  cuneate  fasciculus  end  in  the 
substantia  gelatinosa  (here  known  as  the  nucleus  of  the  spinal  tract  of  the  trigeminal  nerve) 
and  in  the  remnant  of  the  head  of  the  posterior  gray  column  (Fig.  100).  There  are  three 
smaller  gray  masses  within  the  funiculus  cuneatus:  (1)  the  external  round  nucleus,  an  iso- 
lated portion  of  the  substantia  gelatinosa,  near  which  it  is  situated;  (2)  the  internal  round 
nucleus,  more  variable  in  position;  and  (3)  the  accessory  or  lateral  cuneate  nucleus  superficial 
to  the  main  nuclear  mass. 


Funiculus  gracilis 

Nucleus  gracilis 

Spinal  tract  of  trigeminal 
nerve 

Nucleus  of  spina!  tract 
ofN.  V 

Dorsal  motor  nucleus  of    Jjy 
vagus 

Nucleus  of  hypoglossal 
nerve 

Decussation  of  medial 
lemniscus 

Lateral  reticular  nucleus 

Medial  accessory  olivary 
nucleus 

Ventral  external  arcuate 
fibers 


Funiculus  cuneatus 
X ucl ens  cuneatus 
Central  canal 

Internal  arcuate  fibers 
Reticular  substance 

Dorsal  spinocerebellar 

tract 

'^iL___}'entra!  spinocerebellar 

trad 

Ventral  fasciculus 

proprius 

Hypoglossal  nerve 

Pyramid,  corticospinal 
'trad 


Fig.  99. — Section  through  the  medulla  oblongata  of  a  child  at  the  level  of  the  decussation  of  the 
medial  lemniscus.      (Pal-Weigert  method.)      (X  6.) 

The  Medial  Lemniscus  and  its  Decussation. — The  great  majority  of  fibers 
which  arise  from  the  cells  in  the  nucleus  gracilis  and  nucleus  cuneatus  sweep 
ventromedially  in  broad  concentric  curves  around  the  central  gray  substance 
toward  the  median  raphe  (Fig.  99).  As  has  been  stated  on  a  preceding  page, 
these  are  known  as  internal  arcuate  filers,  and  as  they  cross  those  from  the 
opposite  side  in  the  raphe  they  form  the  decussation  of  the  lemniscus  (decussatio 
lemniscorum,  sensory  decussation).  After  crossing  the  median  plane  they  turn 
rostrally  in  the  medial  lemniscus  (fillet),  and  end  in  the  thalamus  (Fig.  235). 
These  longitudinal  fibers  constitute  a  broad  band  which  lies  close  to  the  median 
raphe,  medial  to  the  inferior  olivary  nucleus,  and  dorsal  to  the  pyramids  (Figs. 
96,  97).  By  the  accession  of  additional  internal  arcuate  fibers  this  band  in- 
creases in  size  and  spreads  out  dorsally  until  at  the  level  of  the  middle  of  the 
olive  it  is  separated  from  the  gray  matter  of  the  ventricular  floor  only  by  the 


nil     STRU(   11  RE    OP    Mil     Ml  in  n  \    OBLONGA  I  \ 


fibers  of  the  fasciculus  longitudinalis  medialis  and  the  tectospinal  trad 
101).  The  decussation  of  the  lemniscus  begins  at  the  upper  border  of  the 
decussation  of  the  pyramids,  where  the  sensory  fibers  are  grouped  into  coarse 
bundles  arching  around  the  central  gray  matter  (Fig.  99),  and  extends  as  far  as 
do  the  gracile  and  cuneate  nuclei,  that  is,  to  about  the  middle  of  the  olive.  In 
sections  through  the  lower  half  of  the  olive  the  internal  arcuate  fibers  describe 
broad  curves  through  the  reticular  formation  and  their  decussation  occupi 
considerable  ventrodorsal  extent  of  the  raphe  I  Fig.  101). 


Nerve  cell  in  the  nucleus  cuneatus 


Ramification  of  fibers  front  the  fasciculus  cuneatus 


Nucleus  cum 


Substantia 

linosa 


Fig.  100. — From  a  transverse  section  through  the  medulla  oblongata  of  a  kitten,  to  illustrate 
the  termination  of  the  fibers  of  the  fasciculus  cuneatus,  and  at  a  the  beginning  of  the  internal 
arcuate  fibers.     (Combined  from  drawings  by  Cajal.) 

The  arcuate  fibers  of  the  medulla  oblongata  may  be  separated  into  two 
groups:  those  which  run  through  the  reticular  formation  constitute  the  inter- 
nal arcuate  fibers;  and  those  which  run  over  the  surface  of  the  medulla,  the 
external  arcuate  libers.  The  internal  arcuate  fibers  are  of  at  least  three  kinds: 
(1)  those  described  in  the  preceding  paragraph,  which  arise  in  the  gracile  and 
cuneate  nuclei  and  form  the  medial  lemniscus;  (2)  sensory  libers  of  the  second 
order,  arising  in  the  sensory  nuclei  of  the  cranial  nerves;  and  (3)  olivocerebellar 
fibers,  which  will  be  considered  in  another  paragraph.  Our  knowledge  of  the 
external  arcuate  fibers  is  less  satisfactory.     From  the  nuclei  of  the  posterior  funic- 


140 


THE    NERVOUS    SYSTEM 


uli  and  perhaps  also  from  these  funiculi  themselves  a  group  of  dorsal  external 
arcuate  fibers  make  their  way  to  the  restiform  body  along  the  dorsal  surface  of 
the  medulla  (Fig.  101).  According  to  Cajal  these  fibers  are  well  developed  in 
man,  but  absent  in  the  cat  and  rabbit.  The  ventral  external  arcuate  fibers  are 
said  to  include  a  certain  number  which  arise  in  the  lateral  reticular  and  arcuate 
nuclei  and  run  dorsolaterally  over  the  surface  of  the  medulla  to  reach  the 
cerebellum  by  way  of  the  restifrom  body  (Fig.  104).  The  arcuate  nuclei  are 
small  irregular  patches  of  gray  matter  situated  on  the  ventromedial  aspect  of 
the  pyramid  and  continuous  rostrally  with  the  nuclei  pontis,  with  which  they 


Spinal  vestibular 

nucleus 

Dorsal  external 

arcuate  fibers 

Tractus  solitaries 

and  nucleus 

Nucleus  of 
hypoglossal  nerve 

Internal  arcuate 
fibers 

Dorsal  spinocere- 
bellar tract. 

Medial  longitudinal 
fasciculus 

Ventral  spinocere- 
bellar tract 

Tectospinal  trad 

Medial  lemniscus 

Inferior  olivary 
nucleus 

Hilus  of  olivary 
nucleus 

Pyramid,  cortico- 
spinal tract 


Arcuate  nucleus 


Dorsal  motor 
nucleus  of  vagus 

Nucleus  cuncatus 
Restiform  body 

Central  canal 

Spinal  trad  and 
nucleus  N.  V 

Nucleus  ambiguus 

Reticular   substance 

Lateral  reticular 
nucleus 

Medial  accessory 
olivary  nucleus 

Inferior  olivary 
nucleus 

Hypoglossal  nerve 

Ventral  external 
arcuate  fibers 


Fig.  101.— Section  through  the  medulla  oblongata  of  a  child  at  the  level  of  the  olive.     Pal-Weigert 

method.      (X  6.) 

seem  to  be  homologous  (Figs.  101,  103).  They  probably  receive  fibers  from  the 
cerebral  cortex  by  way  of  the  pyramidal  tracts;  and,  if  so,  the  external  arcuate 
fibers  which  arise  from  them  are  homologous  with  the  transverse  fibers  of  the 
pons. 

Although  the  facts  stated  above  are  pretty  well  established,  only  a  small  part  of  the 
ventral  external  arcuate  fibers  are  thus  accounted  for.  The  origin  and  course  of  the  majority 
of  these  fibers  is  still  obscure.  According  to  Cajal  (1909)  they  arise  from  the  nuclei  of  the 
posterior  funiculus,  curve  ventrally  and  medially  over  the  surface  of  the  medulla  oblongata, 
penetrate  the  pyramids  or  the  anterior  median  fissure,  cross  in  the  median  raphe,  and  join 
the  medial  lemniscus  of  the  opposite   side.      On  the  other  hand,  Edinger  (1911)  gives  to 


THE    STRUC'IM'I     "i     mi.    mi. mi. I. \    OBLONGATA  141 

them  the  name  "tractus  cerebello-tegmentalis  Imllii,"  and  believes  that  thej  from 

the  cerebellum  by  way  of  the  restiform  body,  then  arch  vent  rally  over  the  surface  of  the 
medulla,  penetrate  tin-  pyramid  or  the  anterior  median  fissure,  and  end  in  the  reticular 
formal  ion  of  the  opposite  side  I  Fig.  153).     A<  cording  to  Van  Gehui  hten  (1904  of  the 

ventral  external  arcuate  fibers  arise  from  cells  in  the  reti<  ular  formation  of  the  same  and  the 
opposite  side,  and  run  through  the  restiform  body  to  the  <  erebellum. 

Olivary  Nuclei. — The  oval  prominence  in  the  literal  area  of  the  medulla, 
known  as  the  olive,  is  produced  by  the  presence  just  beneath  the  surface  of  a 
large  gray  mass,  the  inferior  olivary  nucleus,  with  which  there  are  associated 


> -  :-  . 


Fig.  102. — Diagram  to  illustrate  the  structure  of  the  inferior  olivary  nucleus.     (Cajal,  Edinger.) 

two  accessory  olivary  nuclei.  The  inferior  olivary  nucleus  is  very  conspicuous 
in  the  sections  of  this  part  of  the  medulla  (Fig.  101).  It  appears  as  a  broad, 
irregularly  folded  band  of  gray  matter,  curved  in  such  a  way  as  to  enclose  a 
white  core,  which  extends  into  the  nucleus  from  the  medial  side  through  an 
opening,  known  as  the  hilus.  Considered  as  a  whole  this  nucleus  resembles  a 
crumpled  leather  purse,  with  an  opening,  the  hilus,  directed  medially.  Sec- 
tions at  either  end  of  the  nucleus  do  not  include  this  Opening,  and  at  these 
points  the  central  core  of  white  matter  is  completely  surrounded  by  the  gray 
lamina.     The  fibers  which  stream  in  and  out  of  the  hilus  constitute  the  olivary 


142 


THE    NERVOUS    SYSTEM 


peduncle.  The  two  accessory  olives  are  plates  of  gray  substance,  which  in  trans- 
verse section  appear  as  rods.  The  medial  necessary  olivary  nucleus  is  placed  be- 
tween the  hilus  of  the  inferior  olive  and  the  medial  lemniscus,  while  the  dorsal 
accessory  olivary  nucleus  is  located  close  to  the  dorsal  aspect  of  the  chief  nuclear 
mass. 

Structure  and  Connections.— The  gray  lamina  of  the  inferior  olivary  nucleus 
consists  of  neuroglia  and  many  rounded  nerve-cells  beset  with  numerous  short, 
frequently  branching  dendrites,  the  axons  of  which  run  through  the  white  core 
of  the  nucleus  and  out  at  the  hilus  as  olivocerebellar  fibers  (Fig.  102).  About 
these  cells  there  ramify  the  end  branches  of  several  varieties  of  afferent  fibers, 
the  origin  of  which  is  not  well  understood.     Some  come  from  a  tract,  designated 


Fourth  ventricle 
Principal  vestibular  nucleus 
Spinal  vestibular  nucleus 
Nucleus  intcrcalatus—^g; 

Rest  if  or  in  body-h 
Spina!  tract  and     /fellfe'/ 

nucleus  N.  V    [IIP 
Pontobulbar  body  -\^m 

Glossopharyngeal  nerve    *¥ 

Nucleus  ambiguus 
Ventral  spinocerebellar  tract 
Dorsal  accessory  olivary 

nut 
Hilus  of  olivary  nucleus 
Inferior  olivary  nucleus 


Tania  of  fourth  ventricle 


Nucleus  of  liypoglossal  nerve 
Dorsal  motor  nucleus  of  vagus 
Tractus  solitarius  and 

nucleus 
L.  Medial  longitudinal 
%         fasciculus 
a"  Reticular  substance 
m  '■■     Olivocerebellar  fibers 
r~-rZ'<-f '  ~v—  ]'a''us  nerve 

Lateral  reticular  nucleus 


^m^ 


^^y^:?^\C Thalamo-olivary  tract 

||lps3s§§y^  "  ,    --^~ n  Inferior  olivary  nucleus 


0>3 

^P^-w  Medial  lemniscus 
r~— ^-—  Hypoglossal  nerve 

it  - — Pyramid,  corticospinal  tract 

Medial  accessory  olivary  nucleus  '   «:^.    /        s-'-':^-,- 

Ventral  external  arcuate  fibers  ^^^^  ^~**^—-~  Arcuate  nucleus 

Fig.  103. — Section  through  the  medulla  oblongata  of  a  child  at  the  level  of  the  restiform  body. 

Pal-Weigert  method.     (X4.) 

as  the  thalamo-olivary  fasciculus;  but  it  is  not  certain  that  they  have  their 
origin  in  the  thalamus;  quite  possibly  they  come  from  some  other  gray  mass 
in  that  neighborhood.  Another  group  of  fibers,  consisting  chiefly  of  collaterals, 
comes  from  the  ventral  funiculus  of  the  spinal  cord  and  may  be  regarded  as 
ascending  sensory  fibers  (Cajal,  1909).  These  belong  to  the  so-called  spino- 
olivary  fasciculus. 

Olivocerebellar  Fibers. — The  axons  from  the  cells  of  the  inferior  olivary 
nucleus  stream  out  of  the  hilus,  cross  the  median  plane,  and  either  pass  through 
or  around  the  opposite  nucleus.  Here  they  are  joined  by  some  uncrossed  fibers 
from  the  olivary  nucleus  of  the  same  side.  Thence  they  curve  dorsally  toward 
the  restiform  body,  passing  through  the  spinal  tract  of  the   trigeminal   nerve 


Mil     STRUCT!  RE    I  HP     Mil.    Ml  l»i   II  \    I  IBLONG  \  I  \ 


J43 


whirh  becomes  split  up  into  several  bundle-  (Fig.  103).  They  form  an  Im- 
portant group  of  internal  annate  fibers,  which  run  through  the  restiform  bod) 
to  the  cerebellum  and  constitute  the  olivocerebellar  tra<t    Fig.  104 

The  restiform  body  or  inferior  cerebellar  peduncle  is  a  large  and  prominent 

strand  of  fibers  which  gradually  accumulate  along    the  lateral  border  of  the 

caudal  part  of  the  fourth  ventricle.     It  forms  the  floor  of  the  lateral  reo 

that  cavity  and  then  turn-  dorsally  into  the  cerebellum  (Figs.  88,  89,  103).     It 

mposed  for  the  most  part  of  two  large  and  important  fascicles:    (1)  the 


--Restiform  body 
-  -Olivocerebellar  tract 
— Lateral  reticular  nucleus 


Arcuate  fibers  from  arcuate 
nucleus 


Medulla  oblongata 


Spinal  cord — / 


^Dorsal  external  arcuate  fibers 


-  -  Dorsal  spinocerebellar  tract 


Fig.  104. — Diagram  showing  the  fiber  tracts  which  enter  the  restiform  body  from  the  medulla 

oblongata. 

olivocerebellar  fibers,  both  direct  and  crossed,  but  chiefly  from  the  inferior  olivary 
nucleus  of  the  opposite  side;  and  (2)  the  dorsal  spinocerebellar  tract,  from  the 
nucleus  dorsalis  of  the  same  side  of  the  spinal  cord  (Fig.  104).  In  addition, 
there  are  fibers  in  smaller  number  from  other  sources:  (3)  the  dorsal  external 
arcuate  fibers  from  the  gracile  and  cuneate  nuclei  of  the  same  side;  and  fibers 
(4)  from  the  arcuate  nucleus,  (5)  from  the  lateral  reticular  nucleus,  and  possibly 
also  from  other  cells  scattered  through  the  reticular  formation  (Van  Gehuchten. 
1904). 


144  THE    NERVOUS    SYSTEM 

The  dorsal  spinocerebellar  tract  can  readily  be  traced  in  serial  sections  of 
the  medulla  because  the  large,  heavily  myelinated  libers  of  which  it  is  composed 
cause  it  to  be  deeply  stained  by  the  Weigert  technie.  It  can  be  followed  from 
the  spinal  cord  along  the  periphery  of  the  medulla  oblongata  near  the  posterior 
lateral  sulcus.  At  first  it  lies  ventral  to  the  spinal  tract  of  the  trigeminal  nerve 
(Figs.  98,  99).  But  at  the  level  of  the  lower  part  of  the  olive  it  inclines  dorsally, 
passing  over  the  surface  of  the  spinal  tract  of  this  nerve  to  reach  the  restiform 
body  (Fig.  101).  Between  this  tract  and  the  olive  we  find  the  ventral  spino- 
cerebellar tract  also  in  a  superficial  position. 

The  spinal  tract  of  the  trigeminal  nerve  is  formed  by  the  descending  branches 
of  the  sensory  fibers  of  that  nerve.  They  give  off  collateral  and  terminal 
branches  to  a  column  of  gray  matter,  resembling  the  substantia  gelatinosa 

Tractus  solilarius  and  nucleus  s.  e\  ...     ~  Vestibular  nucleus 


Dorsal  motor  nucleus  of  vagus 


\        /  »#$wffiC  ..^Nucleus  cuneatus 


Nucleus  oj  hypoglossal  mrve  /  ■  •/ •■        \    -A',,,/,.,,,  „f  the  spinal  trad  N.  V 

Nucleus  ambiguus ..    .;'•';>>;•."     X     •:^'"'.'^      ,.        ,    ...  in,, 

"i-'vY'  s  .''■•'>•''•■••'• '•^/     Dorsal  spinocerebellar  tract 

Medial  longitudinal  fasciculus  --mk~^\  ^^M--- Spinal  tract  N.  V 
Tectospinal  tract  — 1$*\  tti^mr — Vagus  nerve 

Dorsal  accessory  olivary  nucleus— -p$r V* f*'ty$'<§$ml'.~'      Rubrospinal  tract 

Medial  lemniscus    ■  \     <>%'>    ..,,  "  -        :  •.  '""-Ventral  spinoierebellar  tract 

Medial  accessory  olivary  nucleus  ■         ''^V^^R-  "^^    Spinothalamic  tract 

k-ViAySs  \  "'--••■'-:'.;:' ,•;••\'-'.•  .■'       I  halamo-olivary  tract 


Corticospinal  tract  — 


Inferior  olivary  nucleus 
II  ypoglossal  nerve 


Fig.  105.— Diagram  showing  the  location  of  the  nuclei  and  fiber  tracts  of  the  medulla  oblongata 

at  the  level  of  the  olive. 

Rolandi,  with  which  it  is  directly  continuous,  and  designated  as  the  nucleus 
of  the  spinal  tract  of  the  trigeminal  nerve  (Figs.  92,  98,  99,  101,  103).  The  tract 
lies  along  the  lateral  side  of  the  nucleus  and  is  superficial  except  in  so  far  as  it 
is  covered  by  the  external  arcuate  fibers,  the  dorsal  spinocerebellar  tract,  and  the 
restiform  body.  It  forms  an  elongated  elevation,  the  tuberculum  cinereum  on 
the  surface  of  the  medulla  oblongata  (Fig.  88). 

The  formatio  reticularis  fills  the  interspaces  among  the  larger  fiber  tracts 
and  nuclei.  It  is  composed  of  small  islands  of  gray  matter,  separated  by  fine 
bundles  of  nerve-fibers  which  run  in  every  direction,  but  which  are  for  the 
most  part  either  longitudinal  or  transverse.  It  is  subdivided  into  two  parts. 
The  formatio  reticularis  alba  is  located  dorsal  to  the  pyramid  and  medial  to  the 
root  filaments  of  the  hypoglossal  nerve  and  is  composed  in  large  part  of  longi- 


Tin:    s  l  kl  (  it  ki     iif    mi     mi  im  i  i.\    OBLONGATA  [45 

tudinal  nerve-fibers  belonging  to  the  medial  lemniscus,  tectospinal  tract,  and  the 
medial  longitudinal  fasciculus  (Fig.  105).  The  latter  is  closely  associated  with 
the  vestibular  nerve  and  ran  best  be  described  with  the  central  connectioi 
that  nerve.  The formaiio  reticularis grisea  \>  found  dorsal  t<>  theoliveand  lateral 
to  the  hypoglossal  nerve.  Tn  it  the  nerve-cells  predominate  and  the  trans- 
versely coursing  internal  anuatc  fibers  form  a  conspicuous  feature  It-  longi 
tudinal  fibers,  though  less  prominent,  arc  of  great  Importance.  The  descend- 
ing fibers  include  those  of  the  rubrospinal  tract,  which  tan  be  followed  into  the 
lateral  funiculus  o\  the  spinal  cord,  and  the  thalamo-olivary  fasciculus,  which 
ends  in  the  olive.  Among  the  ascending  libers  are  those  of  the  ventral  and 
dorsal  spinocerebellar,  the  spinothalamic,  and  spinotectal  tracts. 

The  nerve-cells  of  the  reticular  formation  are  scattered  through  the  mesh  of 
interlacing  fibers.  In  certain  localities  they  are  more  closely  grouped  and  form 
fairly-  well-defined  nuclei.  Among  these  we  may  select  two  for  special  atten- 
tion. The  lateral  reticular  nucleus  or  nucleus  of  the  lateral  funiculus  i>  a  long 
column  of  cells  found  along  the  deep  surface  of  the  ventral  spinocerebellar  tract. 
from  which  it  is  said  by  Andre  Thomas  to  receive  afferent  fibers.  At  any  rati'. 
it  receives  fibers  from  the  lateral  funiculus  of  the  spinal  cord  (Cajal,  1909)  and 
sends  its  axons  to  the  cerebellum  by  way  of  the  restiform  body  (Van  Gehuchten, 
1904;  Yagita.  1906).  It  seems,  therefore,  to  be  a  way  station  on  a  sensory 
path  from  the  spinal  cord  to  the  cerebellum.  Some  large  cells  in  the  gray  part 
of  the  reticular  formation  may  be  grouped  together  and  called  the  motor  nucleus 
of  the  tegmentum  (nucleus  magnocellularis  of  Cajal).  Their  axons  become  as- 
cending or  descending  fibers  or  may  bifurcate  into  ascending  and  descending 
branches  within  the  reticular  formation.  Kohnstamm  has  traced  such  fibers 
by  means  of  the  degeneration  method,  and  has  shown  that  they  run  for  the 
most  part  in  a  caudal  direction  and  that  some  of  them  reach  the  cervical  por- 
tion of  the  spinal  cord  (tractus  reticulospinalis — Fig.  115). 

The  nuclei  of  the  cranial  nerves  can  best  be  considered  in  a  separate  chapter. 
At  this  point  it  will  only  be  necessary  to  enumerate  and  locate  the  nuclei  of  those 
nerves  which  take  origin  from  the  medulla  oblongata. 

The  nucleus  of  the  hypoglossal  nerve  contains  the  cells  of  origin  of  the 
motor  fibers  which  compose  that  nerve.  It  forms  a  long  column  of  nerve-cells 
on  either  side  of  the  median  plane  in  the  ventral  part  of  the  gray  matter  sur- 
rounding the  central  canal  and  in  the  floor  of  the  fourth  ventricle  (Figs.  99,  101, 
103).  In  the  latter  region  it  lies  immediately  beneath  that  part  of  the  floor 
which  was  described  in  the  preceding  chapter  under  the  name  of  the  trigonum 


146  THE   NERVOUS   SYSTEM 

hypoglossi  (Fig.  89).  In  reality,  it  corresponds  only  to  the  medial  part  of  this 
eminence,  for  on  its  lateral  side  there  is  found  another  group  of  cells  known  as 
the  nucleus  intercalatus  (Fig.  103).  From  their  cells  of  origin  the  fibers  of 
the  hypoglossal  nerve  stream  forward  through  the  reticular  formation  to  emerge 
at  the  lateral  border  of  the  pyramid. 

The  nucleus  ambiguus  is  a  long  column  of  nerve-cells  which  give  origin  to 
the  motor  fibers  that  run  through  the  glossopharyngeal,  vagus,  and  accessory 
nerves  to  supply  the  striated  musculature  of  the  pharynx  and  larynx.  It  is 
located  in  the  reticular  formation  of  both  the  open  and  the  closed  portions 
of  the  medulla,  ventromedial  to  the  nucleus  of  the  spinal  tract  of  the  trigeminal 
nerve  (Figs.  101,  103). 

The  dorsal  motor  nucleus  of  the  vagus  lies  along  the  lateral  side  of  the 
nucleus  of  the  hypoglossal.  It  occupies  the  ala  cinerea  of  the  rhomboid  fossa 
and  extends  into  the  closed  part  of  the  medulla  oblongata  along  the  lateral 
side  of  the  central  canal  (Figs.  89,  99,  101,  103).  From  the  cells  of  this  nucleus 
arise  the  efferent  fibers  of  the  vagus  nerve  which  innervate  smooth  muscle 
and  glandular  tissue.  The  afferent  fibers  of  the  vagus  and  glossopharyngeal 
nerves  bend  caudally  and  run  within  the  tractus  solitarius. 

The  nucleus  of  the  tractus  solitarius  is  the  nucleus  of  reception  of  the  affer- 
ent fibers  of  the  facial,  glossopharyngeal,  and  vagus  nerves,  i.  e.,  it  contains 
the  cells  about  which  these  afferent  fibers  terminate.  The  tractus  solitarius 
can  be  traced  throughout  almost  the  entire  length  of  the  medulla.  It  decreases 
in  size  as  the  descending  fibers  terminate  in  the  gray  matter  which  surrounds 
it  (Figs.  92,  101,  103). 


CHAPTER  X 

INTERNAL  STRUCTURE  OF  THE  PONS 

The  pons  consists  of  two  portions  which  differ  greatly  in  structure  and  sig- 
nificance. The  dorsal  or  tegmental  part  resembles  the  medulla  oblongata,  of 
which  it  is  the  direct  continuation.  The  ventral  or  basilar  portion  contains 
the  longitudinal  fibers  which  go  to  form  the  pyramids;  but  except  for  these  it  is 
composed  of  structures  which  are  peculiar  to  this  level.  It  is  a  recent  phyletic 
development  and  forms  a  prominent  feature  of  the  brain  only  in  those  mam- 
mals which  have  relatively  large  cerebral  and  cerebellar  hemispheres,  as  might 
be  expected  from  the  fact  that  it  forms  part  of  a  conduction  path  uniting  these 

structures. 

THE  BASILAR  PART  OF  THE  PONS 

The  basilar  portion  of  the  pons  is  the  larger  of  the  two  divisions.  It  is 
made  up  of  fascicles  of  longitudinal  and  transverse  fibers  and  of  irregular  masses 
of  gray  substance,  which  occupy  the  spaces  left  among  the  bundles  of  nerve- 
fibers  and  which  are  known  as  the  nuclei  pontis. 

The  longitudinal  fasciculi  of  the  pons  consist  of  two  kinds  of  fibers:  (1)  those 
of  the  corticospinal  tract,  which  are  continued  through  the  pons  into  the  pyra- 
mids of  the  medulla  oblongata ;  and  (2)  those  which  end  in  the  nuclei  of  the  pons 
and  are  known  as  corticopontine  filers  (Fig.  106).  As  they  pass  through  the  pons 
the  corticospinal  fibers  give  off  collaterals  which  also  end  in  these  nuclei.  The 
longitudinal  fibers  enter  the  pons  at  its  rostral  border  from  the  basis  pedunculi. 
At  first  they  form  on  either  side  a  single  compact  bundle;  but  this  soon  becomes 
broken  up  into  many  smaller  fascicles,  which  are  separated  from  each  other 
by  the  transverse  fibers  and  nuclei  of  the  pons  (Fig.  108).  At  the  caudal  border 
these  bundles  again  become  assembled  into  a  compact  strand,  which  is  con- 
tinued as  the  pyramid  of  the  medulla  oblongata  (Fig.  107).  It  is  evident,  how- 
ever, that  the  volume  of  the  bundles  is  much  greater  at  the  rostral  than  at  the 
caudal  border.  This  is  to  be  explained  by  the  fact  that  the  corticopontine 
fibers  have  left  these  bundles  during  their  passage  through  the  pons  and  have 
come  to  an  end  by  arborization  within  the  nuclei  pontis. 

The  transverse  fibers  are  designated  as  fibre  pontis  and  are  divisable  into  a 
superficial  and  a  deep  group  (librae  pontis  superficiales  and  libra?  pontis  pro- 

147 


148 


THE    NERVOUS    SYSTEM 


fundae).  Those  of  the  superficial  group  lie  ventral  to  the  longitudinal  fasciculi; 
while  the  deep  transverse  bundles  interlace  with  the  longitudinal  ones  or  lie 
dorsal  to  them.  The  majority  of  the  nbra;  pontis  cross  the  median  plane.  These 
are  joined  by  some  uncrossed  fibers  and  gathered  together  on  either  side  of  the 
pons  to  form  a  compact  and  massive  strand,  known  as  the  brat  Mum  pontis  or 
middle  cerebellar  peduncle,  which  curves  dorsally  to  enter  the  white  center  of 
the  cerebellum  (Figs.  88,  108). 

~"  "  Cerebral  cortex 
-  Corticobulbar  tract 
—  J-  Corticospinal  tract 

Temporopontine  tract 

—  Frontopontine  tract 

—  Pons 

—  Cerebellum 
"""*  Nuclei  pontis 

Brachium  pontis 


Lateral  corticospinal  tract 
Ventral  corticospinal  tract 

Fig.  106. — Diagram  of  the  corticopontocerebellar  pathway  and  the  corticospinal  and  cortico- 
bulbar tracts. 

Along  the  rostral  border  of  the  pons  and  brachium  pontis  one  or  two  fiber  bundles  are 
sometimes  found  which  run  an  isolated  course  to  the  cerebellum.  These  are  known  as  the 
fila  lateralia  pontis  or  tcenia  pontis  (Fig.  88).  According  to  Horsley  (1906)  the  constituent 
fibers  arise  from  a  ganglion  situated  caudal  to  the  interpeduncular  ganglion,  decussate  at  once, 
and  end  in  the  cerebellum  in  the  neighborhood  of  the  dentate  nucleus.  Perhaps  they  rep- 
resent slightly  displaced  fibrae  pontis.  Some  of  the  transverse  fibers  on  reaching  the  median 
plane  bend  at  right  angles  and  run  as  fibrae  recta?  toward  the  pars  dorsalis  pontis  (Fig.  108). 
According  to  Edinger  (1911)  these  belong  in  part  at  least  to  the  tractus  cerebellotegmentalis 
pontis,  which  arises  in  the  nuclei  of  the  cerebellum  and  runs  through  the  brachium  pontis 
to  end  in  the  reticular  formation  of  the  opposite  side  (Fig.  153).  Cajal  (1909)  is  doubtful 
about  the  existence  of  such  efferent  fibers  from  the  cerebellum  in  the  brachium  pontis. 

The  nuclei  pontis,   which   are  continuous  with   the  arcuate  nuclei  of  the 
medulla  oblongata,  contain  stellate  nerve-cells  of  varying  size,  the  axons  of 


INTERNAL   STRUCTURE    OF   THE    PONS 


I40 


which  arc  continuous  with  the  fibrae  pontis.  There-  art-  also  some  small  nerve- 
cells  of  dole's  Type  II.  the  short  axons  of  which  end  in  the  adjacenl  gray  mat- 
ter. Within  these  nuclei  terminate  the  fibers  of  the  corticopontine  tracts  and 
Some  collateral-  from  the  corticospinal  fibers.  Collaterals  from  the  medial 
lemniscus  are  also  found  arborizing  in  those  nuclei  of  the  pons  which  lie  im- 
mediately ventral  to  that  bundle.  This  gray  matter,  therefore,  represent-  an 
important  association  apparatus  within  which  there  terminate  fibers  from 
several  different  sources. 

From  what  has  been  said  it  will  be  apparent  that  the  pons  serves  to  estab- 
lish an  important  and  for  the  most  part  crossed  connection  between  the  cere- 
bral hemispheres  and  the  cerebellum,  a  cortico- ponto-ccrcbcllar  path.  The  cor- 
ticopontine fibers  take  origin  from  pyramidal  cells  in  the  frontal  and  temporal 
lobes  and  end  in  the  nuclei  pontis.  Arising  from  the  cells  in  these  nuclei,  most 
of  the  transverse  fibers  cross  the  median  plane  and  reach  the  opposite  cerebellar 
hemisphere  through  the  brachium  pontis  (Fig.  106). 

THE  DORSAL  OR  TEGMENTAL  PART  OF  THE  PONS 

The  dorsal  or  tegmental  part  of  the  pons  (pars  dorsalis  pontis)  resembles  in 
structure  the  medulla  oblongata  (Fig.  108).  On  its  dorsal  surface  there  is  a 
thick  layer  of  gray  matter  which  lines  the  rhomboid  fossa.  Between  this  layer 
and  the  basilar  portion  of  the  pons  is  the  reticular  formation  divided  by  the 
median  raphe  into  two  symmetric  halves.  This  has  essentially  the  same  struc- 
ture here  as  in  the  medulla  oblongata,  and  contains  the  continuation  of  many 
longitudinal  tracts  with  which  we  are  already  familiar.  The  restiform  body  at  first 
occupies  a  position  similar  to  that  which  it  has  in  the  medulla,  along  the  lateral 
border  of  the  rhomboid  fossa;  but  it  soon  bends  dorsally  into  the  cerebellum. 

The  Cochlear  Nuclei. — At  the  point  of  transition  between  the  medulla  and 
pons  the  restiform  body  is  partly  encircled  on  its  lateral  aspect  by  a  mass  of 
gray  matter  formed  by  the  terminal  nuclei  of  the  cochlear  division  of  the  acoustic 
nerve  (Fig.  107).  There  may  be  distinguished  a  dorsal  and  a  ventral  cochlear 
nucleus  at  the  dorsal  and  ventral  borders  of  the  restiform  body.  Within  these 
nuclei  the  fibers  of  the  cochlear  nerve  end;  while  those  of  the  vestibular  nerve 
plunge  into  the  substance  of  the  pons  ventromedially  to  the  restiform  body  to 
reach  the  floor  of  the  fourth  ventricle  (Fig.  134).  Fibers  from  the  dorsal  cochlear 
nucleus  run  medially  upon  the  floor  of  the  fourth  ventricle  in  the  stria?  medullares 
(Fig.  89) ,  and  sinking  into  the  tegmentum  join  the  fibers  from  the  ventral  coch- 
lear nucleus  in  the  trapezoid  body. 


IS© 


THE    NERVOUS    SYSTEM 


The  trapezoid  body  (corpus  trapezoideum),  which  in  most  mammals  appears 
on  the  surface  of  the  medulla  near  the  border  of  the  pons  (Fig.  83),  is  covered 
in  man  by  the  enlarged  pars  basalis  pontis.  In  sections  through  the  more  caudal 
portions  of  the  pons  the  trapezoid  body  forms  a  conspicuous  bundle  of  trans- 
verse fibers  in  the  ventral  portion  of  the  reticular  formation  (Fig.  108).  The 
fibers  are  associated  with  the  terminal  nuclei  of  the  cochlear  nerve,  especially 
the  ventral  one,  and  with  the  superior  olivary  nucleus,  around  the  ventral  border 
of  which  they  swing  in  such  a  way  as  to  form  a  bay  for  its  reception.  Farther 
medialward  they  pass  through  the  medial  lemniscus  at  right  angles  to  its  con- 


Dorsal  cochlear  nucleus 


Fourth  ventricle 
Strice  medullares 


Vent,  spinocerebellar  tract 

Vent,  external  arcuate  fibers 
Medial  lemniscus 


Nucleus  of  emineniia  teres 

Principal  vestibular  nucleus 

Lateral  vestibular 
nucleus 

Nucleus  of  tractus 
solitarius 

Glossopharyngeal 
*C     nerve 
,  }X  Dorsal  cochlear 
[    ||        nucleus 

'stiform  body 
entral  cochlear 
nucleus 

Spinal  trad  and 
nucleus  N.  V 

Trapezoid  body 
Pontobulbar  body 
Medial  longitudinal  fasciculus 
Thalamo-olivary  tract 
Inferior  olivary  nucleus 
Pyramid,  corticospinal  tract 
Arcuate  nucleus 


Foramen  caecum  Pons 

Fig.  107.— Section  through  caudal  border  of  the  pons  and  the  cochlear  nuclei  of  a  child.     Pal- 

Weigert  method.     (X  4.) 

stituent  fibers  and  decussate  in  the  median  raphe.  The  trapezoid  body  de- 
scribes a  curve  with  convexity  directed  rostrally  as  well  as  ventrally,  and  as  a 
result  its  lateral  portions  are  seen  best  in  sections  through  the  lower  border 
of  the  pons  (Fig.  107),  while  the  rest  of  it  is  in  evidence  in  sections  at  a  higher 
level  (Fig.  108).  Arising  from  the  ventral  nucleus  of  the  cochlear  nerve  (Fig. 
107)  these  fibers  pass,  with  or  without  interruption  in  the  superior  olivary 
nucleus,  across  the  median  plane  (Fig.  108);  and,  on  reaching  the  lateral  border 
of  the  opposite  superior  olivary  nucleus,  they  turn  rostrally  to  form  a  longi- 
tudinal band  of  fibers  known  as  the  lateral  lemniscus  (Fig.  110).     This  is  a 


INTERNAL    STKIVTI '  k  K    OF     Mil.    I'OXS 


151 


part  of  the  central  auditory  pathway,  the  connections  of  which  are  represented 
diagrammatically  in  Fig.  134. 

The  superior  olivary  nucleus  is  a  small  mass  of  gray  matter  located  in  the 
ventrolateral  portion  of  the  reticular  formation  of  the  pons  in  close  relation  to 
the  trapezoid  body  and  not  far  from  the  rostral  pole  of  the  inferior  olivary  nucleus 
(Figs.  108,  110).  It  consists  of  two  or  three  separate  but  closely  associated 
nuclear  masses  composed  of  small  fusiform  nerve-cells,  among  which  there 
ramify  collaterals  from  the  fibers  of  the  trapezoid  body.     From  the  dorsal  aspect 


Superior  vestibular  nucleus 

I  v-f  •v.M'v. 


A  bducens  nerve 

Genu  of  facial  N .  / 

Medial  longitudinal 
fasciculus 


Fourth  ventricle 


Restiform  body 
Brachium  pontis 

Nucleus  of  abducens  N. 

Facial  nerve 

Spinal  tract  and  nu- 
cleus N.  V 

Nucleus  of  facial  N. 

Thalamo-olivary  tract 

Superior  olivary  huch  u  s 

Trapezoid  body  and 
medial  lemniscus 

Deep  stratum  of  pons 

Corticospinal  and  cortico- 
pontine tracts 


Nuclei  pontis 
Superficial  stratum  of  pons 


Fig.  108. — Section  through  the  pons  of  a  child  at  the  level  of  the  facial  colliculus.     Pal-Weigert 

method.     (X  4.) 

of  this  nucleus  a  bundle  of  fibers,  known  as  the  peduncle  of  the  superior  olive, 
makes  its  way  toward  the  nucleus  of  the  abducens  nerve,  and  it  may  be  that 
some  of  these  fibers  enter  the  medial  longitudinal  bundle  (Fig.  124). 

The  nuclei  of  the  vestibular  nerve  lie  in  the  floor  of  the  fourth  ventricle, 
where  they  occupy  a  field  with  which  we  are  already  familiar,  namely,  the  area 
acustica  (Fig.  89).  The  vestibular  fibers  on  approaching  the  rhomboid  fossa 
divide  into  ascending  and  descending  branches,  and  terminate  in  four  nuclear 
masses:  (1)  the  medial  (dorsal  or  principal)  vestibular  nucleus  (Figs.  103,  107), 
(2)  the  lateral  vestibular  nucleus  of  Deiters  (Fig.  107),  (3)  the  superior  vestibular 


i=;2 


THE    NERVOUS    SYSTEM 


nucleus  of  Bechterew  (Fig.  108),  (4)  the  spinal  or  descending  vestibular  nucleus 
(Fig.  103).     These  are  represented  diagrammatically  in  Fig.  136. 

The  medial  longitudinal  fasciculus  is  an  important  bundle  which  extends 
from  near  the  floor  of  the  third  ventricle  to  the  spinal  cord,  and  is  especially 
concerned  with  the  reflex  control  of  the  movements  of  the  head  and  eyes.  A 
large  proportion  of  its  fibers  are  derived  from  the  lateral  vestibular  nucleus. 


.1/.  rectus  medialis 
M .  rectus  lateralis 

Nucleus  of  med.  long.  fasc. 

Nucleus  of  oculomotor  nerve 
Nucleus  of  trochlear  nerve 


Nucleus  of  abducens  nerve 
Medial  longitudinal  fasciculus 

Lateral  vestibular  nucleus 


Vestibular  nerve 


Fig.   109. — Diagram  showing  the  connections  of  the  medial  longitudinal  fasciculus.      (Modified 

from  Yilliger.) 

From  this  origin  the  fibers  pass  horizontally  through  the  reticular  formation  to 
the  median  longitudinal  fasciculus  of  the  same  or  the  opposite  side,  and  there 
divide  into  ascending  and  descending  branches  (Fig.  109).  The  former  terminate 
in  the  nuclei  of  the  oculomotor,  trochlear,  and  abducens  nerve,  the  latter  in 
the  nucleus  of  the  spinal  accessory  nerve  and  in  the  columna  anterior  of  the 
cervical  portion  of  the  spinal  cord.     In  this  way  there  is  established  a  path  for 


IN  I  I.KN AX    STRUCTCTRE    OF    THE    PONS 


153 


the  reflex  control  of  the  movement  of  the  head,  neck,  and  eyes  in  response  to 
Stimulation  of  the  nerve  endings  in  the  semicircular  canals  of  the  ears.  Another 
important  group  of  fibers  within  this  fasciculus  takes  origin  from  a  collection 
of  cells  situated  in  the  hypothalamus  ju>t  rostral  to  the  red  nucleus,  which 
Cajal  (1911)  has  called  the  interstitial  nucleus}  but  which  might  properly  be 
designated  as  the  nucleus  of  the  medial  longitudinal  fasciculus.  According  to 
Cajal  the  fascicle  also  contains  ascending  fibers  from  the  ventral  fasciculus 
proprius  of  the  spinal  cord.  Still  other  fibers  serve  to  connect  the  nuclei  of  the 
oculomotor  and  abducens  nerves. 

The  medial  longitudinal  fasciculus  is  continued  into  the  ventral  fasciculus 
proprius  of  the  spinal  cord.  These  fibers  are  displaced  dorsolaterally  by  the 
decussation  of  the  pyramids  (Fig.  98)  and  then  still  farther  dorsally  by  the 
decussation  of  the  lemniscus  (Fig.  99)  until  they  come  to  lie  in  the  most  dorsal 
part  of  the  substantia  reticularis  alba  (Fig.  101),  which  position  they  occupy 
throughout  the  remainder  of  their  course.  The  fasciculus  is  found  ventral  to 
the  nucleus  of  the  hypoglossal  nerve  (Fig.  103)  and  in  close  apposition  to  the 
nuclei  of  the  three  motor  nerves  of  the  eye  (Figs.  108,  114,  116). 

The  medial  lemniscus  can  also  be  traced  within  the  reticular  formation  from 
the  medulla  into  and  through  the  pons.  But  this  broad  band  of  longitudinal 
fibers,  which  was  spread  out  along  the  median  raphe  in  the  medulla,  shifts 
ventrally  in  the  pons,  assuming  first  a  somewhat  triangular  outline  and  a  ven- 
tromedian  position  (Fig.  107);  then  by  shifting  farther  lateralward  it  takes 
again  the  form  of  a  flat  band  (Figs.  108,  110).  But  now  it  is  compressed  ven- 
trodorsally  and  occupies  the  ventral  part  of  the  reticular  formation,  its  fibers 
crossing  those  of  the  trapezoid  body  at  right  angles.  It  must  not  be  forgotten 
that  the  medial  lemniscus  is  composed  of  longitudinal  fibers,  and  it  is  by  the 
gradual  shifting  of  these  that  the  bundle  as  a  whole  changes  shape  and  posi- 
tion. As  it  is  displaced  ventrally  it  separates  from  the  medial  longitudinal 
bundle,  which  retains  its  dorsal  position. 

The  motor  nucleus  of  the  facial  nerve  occupies  a  position  in  the  reticular 
formation  dorsal  to  the  superior  olive  (Fig.  108).  It  is  an  oval  mass  of  gray 
matter,  which  extends  from  the  lower  border  of  the  pons  to  the  level  of  the 
facial  colliculus,  and  contains  the  cells  of  origin  of  the  fibers  which  innervate 

1  The  interstitial  nucleus  of  Cajal  must  not  be  confused  with  the  nucleus  of  the  posterior 
commissure  of  Darkschewitsch  which  lies  in  the  mesencephalon  just  rostral  to  the  oculomotor 
nucleus  and  which,  according  to  Cajal,  may  or  may  not  send  fibers  into  the  medial  longitudinal 
bundle. 


154 


THE    NERVOUS    SYSTEM 


the  platysma  and  muscles  of  the  face.  These  fibers  emerge  from  the  dorsal 
surface  of  the  nucleus  and  run  dorsomedially  toward  the  floor  of  the  fourth 
ventricle.  Somewhat  widely  separated  at  first,  they  become  united  on  the 
medial  side  of  the  abducens  nerve  into  a  compact  strand,  which  as  the  genu  of 
I lie  facial  nerve  partly  encircles  this  nucleus,  and  which  then  runs  ventrolaterally 
between  the  spinal  tract  of  the  trigeminal  nerve  and  its  own  nucleus  toward 
its  exit  from  the  brain  (Figs.  108,  124). 


Anterior  medullary  velum 


Medial  longitudinal  fasciculus^ 

Ventral  spinocerebellar  tract 


Trapezoid  b 
Superior  ol 

Lateral  lemniscu 
Brachium  pontis 


Fourth  ventricle 

Brachium  conjunctivum 

Mesencephalic  root  of  trigem- 
inal nerve 
Motor  nucleus  of  trigeminal 
nerve 
Sensory  nucleus  of  trigem- 
inal nerve 


Medial  lemniscus 
Superficial  stratum  of  pon 


Trigeminal  nerve 
Corticospinal  and  cortico- 
pontine tracts 

pontis 


Fig.  110. — Section  through  the  pons  of  a  child  at  the  level  of  the  motor  nucleus  of  the  trigeminal 

nerve.     Pal-Weigert  method.     (X  4.) 

The  nucleus  of  the  abducens  nerve  along  with  the  genu  of  the  facial  pro- 
duces a  rounded  elevation  in  the  rhomboid  fossa,  known  as  the  facial  colliculus 
(Figs.  89,  108).  It  is  a  spheric  mass  of  gray  matter  containing  the  cells  of  origin 
of  the  fibers  which  innervate  the  lateral  rectus.  These  emerge  from  the  dorsal 
and  medial  surfaces  of  the  nucleus  and  run  ventrally  more  or  less  parallel  to  the 
median  raphe  toward  their  exit  at  the  lower  border  of  the  pons. 

The  Nuclei  of  the  Trigeminal  Nerve. — In  transverse  section  through  approxi- 


IN  l  I  K\  \1.   STRU<   11  RE    I  >]     I  111.    PONS  j-- 

mately  the  middle  of  the  pons  we  encounter  the  fibers  of  the  trigeminal  nerve 
ami  two  associated  masses  of  gray  matter,  the  motor  and  main  sensory  nuclei 
of  that  nerve  (Fig.  110).    These  are  located  close  together  in  tin   dorsolateral 

part  of  the  reticular  formation  near  the  groove  between  the  middle  an. I  supe- 
rior cerebellar  peduncles.  ( If  the  two,  the  sensory  nucleus  is  the  more  superfi<  ial. 
It  is.  in  reality,  not  a  new  structure,  but  rather  the  enlarged  rostral  extremity 

of  the  column  of  gray  matter  which  we  have  followed  upward  from  the  sub- 
stantia gelatinosa  Rolandi  of  the  spinal  cord  and  have  designated  as  the  nucleus 
of  the  spinal  tract  of  the  trigeminal  nerve  (Figs.  98,  101).    On  its  medial  side  is 

found  the  motor  nucleus,  a  large  oval  mass  of  gray  matter  from  the  cells  of  which 
arise  the  motor  fibers  for  the  muscles  of  mastication.  Some  of  the  fibers  of  the 
trigeminal  nerve,  passing  between  these  two  nuclei,  are  continued  as  the  mesen- 
cephalic root  of  the  trigeminal  nerve  (Figs.  110,  111).  Reaching  the  gray  matter 
in  the  lateral  wall  of  the  rostral  part  of  the  fourth  ventricle,  this  bundle  of  fibers 
turns  rostrally  along  the  medial  side  of  the  brachium  conjunctivum  (Fig.  112). 
It  extends  into  the  mesencephalon  in  the  lateral  part  of  the  gray  matter  which 
surrounds  the  cerebral  aqueduct  (Fig.  114).  The  fibers  of  this  root  take  origin 
from  unipolar  cells  scattered  along  its  course  and  known  as  the  mesencephalic 
nucleus  of  the  trigeminal  nerve. 

It  will  be  apparent  from  this  description  that  there  are  four  nuclear  masses 
associated  with  the  trigeminal  nerve,  namely,  the  nucleus  of  the  spinal  tract, 
and  the  main  sensory,  motor,  and  mesencephalic  nuclei.  The  relations  which 
each  of  these  groups  of  cells  bear  to  the  fibers  of  the  trigeminal  nerve  are  illus- 
trated in  Fig.  111.  Note  that  those  fibers  which  arise  from  cells  in  the  semi- 
lunar ganglion  divide  into  short  ascending  and  long  descending  branches.  The 
former  end  in  the  main  sensory  nucleus;  while  the  latter  run  in  the  spinal  tract 
of  the  trigeminal  nerve  and  end  in  the  nucleus  which  accompanies  it. 

The  brachium  conjunctivum  or  superior  cerebellar  peduncle  (Fig.  89)  is  seen 
in  sections  through  the  rostral  half  of  the  pons,  where  it  enters  into  the  lateral 
boundary  of  the  fourth  ventricle.  It  is  a  large  strand  of  fibers  which  runs  from 
the  dentate  nucleus  of  the  cerebellum  to  the  red  nucleus  of  the  mesencephalon 
(Fig.  115).  As  it  emerges  from  the  white  center  of  the  cerebellum  this  brachium 
is  superficially  placed,  with  its  ventral  border  resting  on  the  tegmental  portion 
of  the  pons  (Fig.  110).  To  its  dorsal  border  is  attached  a  thin  plate  of  white 
matter,  the  anterior  medullary  velum,  which  roofs  in  the  rostral  part  of  the 
fourth  ventricle.  As  the  brachium  ascends  toward  the  mesencephalon  it  -inks 
deeper  and  deeper  into  the  dorsal  part  of  the  pons  until  it  is  entirely  submerged 


i<6 


THE    NERVOUS    SYSTEM 


(Fig.  112).     Near  the  rostral  border  of  the  pons  it  assumes  a  crescentic  outline 
and  lies  in  the  lateral  part  of  the  reticular  formation.     From  its  ventral  border 


Fig.  111. — Diagram  of  the  nucle^and  central  connections  of  the  trigeminal  nerve:  A,  Semi- 
lunar ganglion;  B,  mesencephalic  nucleus,  N.  V.;  C,  motor  nucleus,  N.  V.;  D,  motor  nucleus,  N. 
VII;  E,  motor  nucleus,  X.  XII ;  F,  nuchus  of  the  spinal  tract  of  X.  V;  G,  sensory  fibers  of  the  sec- 
ond order  of  the  trigeminal  path;  a,  ascending  and  b,  descending  branches  of  the  sensory  fibers, 
X.  V;  c,  ophthalmic  nerve;  d,  maxillary  nerve;  e,  mandibular  nerve.     (Cajal.) 

fibers  stream  across  the  median  plane,  decussating  with  similar  fibers  from  the 
opposite  side.     This  is  the  most  caudal  portion  of  the  decussation  of  the  brachinm 


I\  I  KRXAL    SUM  ill   RE    OF    THE    PONS 


157 


conjunctivum,  which  increases  in  volume  as  it  i>  followed  rostrally,  reachin 
maximum  in  the  mesencephalon  at  the  level  <>!'  the  interior  colliculi.     In  this 
decussation  the  fibers  of  the  brachium  undergo  a  complete  crossing. 

The  ventral  spinocerebellar  tract,  which  has  made  it>  way  through  the  retic- 
ular formation    of   the   pons,  turns  dorsolaterally  near   the   rostral  <n<l  of  the 
pons,  winds  around  the  brachium  conjunctivum,  and  enters  the  anterior  medul 
lary  velum,  in  which  it  descends  to  the  vermis  of  the  cerebellum  (Figs.  110. 
110). 


Fourth  ventricle 
Dorsal  longitudinal  fasciculus 
Media!  longitudinal  fasciculus 
Thalamo-olivary  tract  \S 
Brachium  conjunctivum  JJBB. 

Decussation  of  brachium 
conjunctivum 


Trochlear  nerve 

Mesencephalic  root  of  trigeminal 
^"     nerve 

jte^V' Lateral 'lemniscus  ami  nucleus 
~*ar — -  Dorsal  nucleus  of  tegmentum 

Ventral  nucleus  of 

tegmentum 

■.  'JMk\ — Nucleus  centralis  superior 
Medial  lemniscus 
Pons 


^z<s//y/     Jl  // ,    \ 

Fig.  112. — Dorsal  half  of  a  section  through  the  rostral  part  of  the  human  pons.  The  index 
line  to  the  mesencephalic  root  of  the  trigeminal  nerve  does  not  quite  reach  that  structure.  Pal- 
Weigert  method. 

The  lateral  lemniscus  is  an  important  tract  of  fibers  which  we  have  already 
traced  from  the  cochlear  nuclei  by  way  of  the  trapezoid  body  and  striae  medul- 
lars acusticae.  It  first  takes  definite  shape  about  the  middle  of  the  pons,  where 
it  is  situated  lateral  to  the  medial  lemniscus  (Fig.  110).  As  it  ascends  it  becomes 
displaced  dorsolaterally  until  it  occupies  a  position  on  the  lateral  aspect  of  the 
brachium  conjunctivum  (Fig.  112).  In  this  position  there  is  developed  in  con- 
nection with  it  a  collection  of  nerve-cells,  the  nucleus  of  the  lateral  lemniscus, 
to  which  its  fibers  give  off  collaterals. 


CHAPTER  XI 

THE  INTERNAL  STRUCTURE  OF  THE  MESENCEPHALON 

A  diagram  of  a  transverse  section  through  the  rostral  part  of  the  mesen- 
cephalon will  make  clear  the  relation  of  the  various  parts  of  the  midbrain  to 
each  other  (Fig.  113).  The  cerebral  aqueduct  is  surrounded  by  a  thick  lamina 
of  gray  matter,  the  central  gray  stratum  (stratum  griseum  centrale).  Dorsal  to 
this  lies  the  lamina  quadrigemina,  a  plate  of  mingled  gray  and  white  matter 
which  bears  four  rounded  elevations,  the  corpora  quadrigemina.     The  ventral 

part  of   the  midbrain   is  formed  by 
Lamina  quadrigemina  ...  the  cereorai  peduncles,  each  of  which 

Cerebral  aqueduct    ---/ /^     ~^N         is   separated   into    two    parts    by   a 

Central  gray  stratum. ./     ...t'V    )         lamina  of  pigmented  gray  substance, 

Tegmentum   l \  /  L   °  •* 

TfSjjf  known     as     the     substantia     nigra. 

Dorsal  to  this  the  peduncle  consists 
of     reticular    formation     continuous 

Basis  pedunetdi-^K.             ^W  ^HC--  W^n  tnat  °^   tne   Pons  anc^  known  as 
Substantia  nigra--'^^. — /              ^  the  tegmentum.     Ventral  to  the  sub- 
Fig.  1 13.-Diagrammatic  cross-section  through  stantia  nigra  is  a  thick  plate  of  longitu- 
the  human  mesencephalon. 

dinal  fibers,  called  the  basis  pedunculi, 
composed  of  fibers  which  are  continuous  with  the  longitudinal  fasciculi  of  the 
pons. 

The  Tegmentum. — The  dorsal  portion  of  the  pons  is  directly  continuous 
with  the  tegmentum  of  the  mesencephalon.  Both  are  composed  of  reticular 
formation,  consisting  of  interlacing  longitudinal  and  transverse  fibers  grouped 
in  fine  bundles  and  separated  by  minute  masses  of  gray  substance,  in  which  are 
embedded  important  nuclei  and  fiber  tracts.  In  the  caudal  part  of  the  mid- 
brain and  the  rostral  part  of  the  pons  are  four  cellular  masses  the  locations  of 
which  are  indicated  in  Fig.  112.  They  are  the  dorsal  nucleus  of  the  raphe,  the 
superior  central  nucleus,  the  ventral  tegmental  nucleus,  and  the  dorsal  tegmental 
nucleus.  The  latter  is  a  collection  of  small  cells  in  the  central  gray  substance,  sep- 
arated from  the  ventral  tegmental  nucleus  by  the  medial  longitudinal  bundle. 
Both  the  ventral  and  dorsal  tegmental  nuclei  receive  fibers  from  the  mammillary 
body  (tractus  mamillotegmentalis) ,  and  within  the  dorsal  one  there  also  ter- 
158 


1111.    [NTERNAL  STRUCTURE    01     nil.    MESENCEPHALON 


minute  fibers  from  the  interpeduncular  ganglion  (Fig.  211).  The  tegmentum 
contains  many  longitudinal  fiber  tracts  which  are  continued  into  it  from  the  dor- 
sal part  of  the  pons.  The  most  conspicuous  of  these  is  the  brackium  conjunc- 
tivum. 

The  Decussation  of  the  Brachia  Conjunctiva. — In  the  sections  of  the  p 
saw  that,  as  the  brachia  conjunctiva  ascend  toward  the  mesencephalon,  they 
sink  deeper  and  deeper  into  the  pars  dorsalis  pontis  I  Fig.  112).     When  they 
reach  the  level  of  the  inferior  colliculi  of  the  corpora  quadrigemina  they  are 


Aqueduct  of  eerebru 

Mesencephalic  root  of  X 

Medial  longitudinal 
fasciculus 
Decussati  >>i  ofbrachi 
conjunctiv 

Interpeduncular  fos 
Substantia,  nigra 


issurc  of  inft  rior  colliculi 

crior  quadrigeminal  brat  hium 

uclcus  of  inferior  colliculus 
Lateral  lemniscus 
Trochlear  m 
Thalamo-olivary  trad 
—Nucleus  of  trochlear  nerve 


^   Medial  lemniscus 

Basis  pedunculi 

Posterior  perforated  substance 

Pons 


Fig.  114.— Section  through  the  mesencephalon  of  a  child  at  the  level  of  the  inferior  colliculus. 

Pal-Weigert  method.      (X  4.) 

deeply  placed  in  the  tegmentum;  and  here  they  cross  the  median  plane  in  the 
decussation  of  the  brackium  conjunctivum  (Fig.  114).  After  crossing,  each  brach- 
ium  turns  rostrally  and  forms  a  rounded  bundle  of  ascending  fibers,  which  al- 
most at  once  comes  into  relation  with  the  red  nucleus  (Fig.  116).  Many  of  the 
fibers  enter  this  nucleus  directly,  while  others  are  prolonged  over  its  surface  to 
form  a  capsule  that  is  best  developed  on  its  medial  surface.  While  the  majority 
of  these  fibers  ultimately  end  in  the  red  nucleus,  some  reach  and  end  within  the 
ventral  part  of  the  thalamus  (Fig.  115).  By  way  of  summary  we  may  repeat 
that  the  fibers  of  the  brachium  conjunctivum.  or  at  least  the  greater  part  of  them, 


i6o 


THE    NERVOUS    SYSTEM 


arise  in  the  dentate  nucleus  of  the  cerebellum;  they  cross  the  median  plane 
in  the  tegmentum  at  the  level  of  the  inferior  colliculi  and  end  either  in  the  red 
nucleus  or  in  the  thalamus. 

According  to  Cajal  (1911)  the  fibers  of  the  brachium  conjunctivum  give  off  two  sets  of 
descending  branches,  which  he  has  seen  in  Golgi  preparations  of  the  mouse,  rabbit,  and  cat. 
The  first  group  are  collaterals  given  off  as  the  brachium  enters  the  dorsal  part  of  the  pons 
and  before  its  decussation  (Fig.  115).  They  descend  into  the  pons  and  medulla  oblongata 
and  constitute  a  direct  descending  tract  from  the  dentate  nucleus  of  the  cerebellum  to  the 
reticular  formation  of  the  pons  and  medulla  oblongata.     The  second  group  of  descending 

From  frontal  lobe  and  corpus  striatum 
"  Thalamus 


Rubrospinal  trad  s 
Rubroreticular  tract 


Red  nucleus 

Brachium  conjunctivum 
Dentate  nucleus 

\ 


Pons 
Rubrospinal  tract 

Medulla  oblongata 
-Reticulospinal  tract 


Spinal  cord 


m 

Fig.  115. — Diagram  showing  the  connections  of  the  red  nucleus:  A,  Ventral  tegmental 
decussation;  B,  decussation  of  the  brachium  conjunctivum;  Cand  D,  descending  fibers  from  bra- 
chium conjunctivum,  before  and  after  its  decussation  respectively. 

branches  is  formed  by  the  bifurcation  of  the  fibers  of  the  brachium  conjunctivum  just  beyond 
the  decussation,  and  constitute  a  crossed  descending  tract  from  the  dentate  nucleus,  which 
can  be  followed  by  degeneration  methods  through  the  reticular  formation  of  the  brain  stem 
and  probably  into  the  anterior  and  lateral  funiculi  of  the  spinal  cord  (Fig.  115). 

The  red  nucleus  (nucleus  ruber)  is  a  very  large  oval  mass  of  gray  matter, 
which  in  the  fresh  brain  has  a  pink  color.  It  is  located  on  the  path  of  the  brach- 
ium conjunctivum  in  the  rostral  part  of  the  tegmentum  (Fig.  116).  In  trans- 
verse sections  it  presents  a  circular  outline  and  can  be  followed  from  the  level 
of  the  inferior  border  of  the  superior  colliculus  into  the  hypothalamus.  In  its 
caudal  portion  it  contains  great  numbers  of  libers  derived  from  the  brachium 


THE  DJTERNAL  STRUCTURE  01  CHE  MESENCEPHALON         1O1 

conjunctivum,  and  stains  deeply  in  Weigerl  preparations,  hut  farther  rostrally 
these  fibers  are  less  numerous  and  the  nucleus  take-  on  more  and  more  theap 
pearance  of  gray  substance. 

Afferent  fibers  reach    the   red    nucleus   chiefly  through   the  brachium   con 
junctivum,  but  it  also  receives  fibers  from  the  cerebral  cortex  of  the  frontal 
lobe  and  other-  from  the  corpus  striatum  (Fig.  115).    These  descending  fibers 
help  to  form  the  capsule  of  the  nucleus  and  are  most  abundant  along  its  medial 
surface. 

Efferent  Fibers.  From  the  cells  of  the  red  nucleus  arise  the  fiber-  of  the 
rubrospinal  tracts  which  after  crossing  the  median  plane  descend  into  the  spinal 
cord.  Other  cells  give  origin  to  liber-,  which  decussate  along  with  those  of  the 
rubrospinal  tract  and  terminate  in  the  nuclei  of  the  reticular  formation  and  in 
the  nucleus  of  the  lateral  lemniscus.  These  form  the  tractus  rubroreticular 
(Fig.  115).     Other  fibers  from  the  red  nucleus  reach  the  thalamus. 

The  nerve-cells  which  are  found  in  the  red  nucleus  vary  greatly  in  size.  The  smaller 
ones  have  the  character  of  the  cells  of  the  reticular  formation  and  send  their  axons  into  the 
tegmentum  of  the  same  and  the  opposite  side.  Another  group  of  very  large  cells  furnishes 
the  axons  that  constitute  the  rubrospinal  tract.  This  collection  of  large  cells  is  phylogenetic- 
ally  the  older  and  forms  the  chief  part  of  the  red  nucleus  in  the  lower  mammals.  But  in 
man,  where  the  two  parts  are  rather  sharply  differentiated,  the  chief  mass  is  composed  of 
the  smaller  cells. 

The  red  nucleus  may  be  regarded  as  an  especially  highly  developed  portion  of  the  motor 
nuclei  of  the  tegmentum.  In  the  lower  mammals  it  serves  as  a  center  through  which  the 
cerebellum  can  influence  the  motor  functions  of  the  spinal  cord  and  medulla  oblongata. 
In  man  it  has  the  same  function,  but  is  also  more  closely  linked  with  the  reticular  formation 
of  the  pons  by  way  of  the  rubroreticular  tract.  It  is  a  significant  fact  that  in  man  where 
the  rubrospinal  tract  is  relatively  small  the  rubroreticular  tract  is  especially  well  developed. 
This  suggests  the  possibility  that  impulses  from  the  red  nucleus  may  be  relayed  through  the 
reticular  nuclei  of  the  pons  to  the  spinal  cord  (Fig.  115). 

The  Tegmental  Decussations. — At  the  level  of  the  superior  colliculus  and 
between  the  two  red  nuclei  the  median  raphe  presents  an  unusual  number  of 
crossing  fibers  (Fig.  116).  Among  these  are  included  the  dorsal  tegmental  de- 
cussation (fountain  decussation  of  Meynert)  and  the  ventral  tegmental  decussa- 
tion (fountain  decussation  of  Forel).  The  latter  is  composed  of  fibers  from  the 
red  nucleus,  which,  after  crossing  the  median  plane,  descend  through  the  brain 
stem  into  the  lateral  funiculus  of  the  spinal  cord  as  the  rubrospinal  tract  I  Fig. 
115).  The  dorsal  tegmental  decussation  is  composed  of  fibers  which  arise  in  the 
superior  colliculi  of  the  corpora  quadrigemina,  sweep  in  broad  curves  around  the 
central  gray  stratum,  and  after  crossing  the  median  plane  in  the  dorsal  part  of 
the  raphe,  go  to  form  the  tectobulbar  and  tectospinal  tracts. 


162 


THE    NERVOUS    SYSTEM 


The  median  longitudinal  fasciculus  is  more  conspicuous  in  the  mesencephalon 
than  in  other  parts  of  the  brain  stem,  but  it  occupies  the  same  relative  position, 
that  is,  near  the  median  plane  close  to  the  central  gray  matter.  At  the  level  of 
the  superior  colliculus  it  forms  a  rather  broad  obliquely  placed  lamina,  extending 
dorsolaterally  from  the  median  raphe,  which  together  with  the  corresponding 
lamina  of  the  opposite  side  produces  in  transverse  sections  a  V-shaped  figure 
(Fig.  116).  The  apex  of  this  V  is  directed  ventrally;  and  included  between  its 
two  limbs  are  the  oculomotor  nuclei.     At  the  level  of  the  inferior  colliculi  the 


Stratum  zonule 

Stratum  griscum-y£    4g 

Stratum  optic  urn   •    d 

Stratum  lemnisci 

Stratum  profundum— 

Aqueduct  of  cere-   ^p 

brum  .  '•   . 

Medial  lemnis 

CUS  ffi 


Superior  colliculus 

Nucli  us  of  oculomotor  nerve 
Medial  longitudinal  fasciculus 
Thalamo-olivary  tract 

Inf.  quadrigemina!  brack. 
Med.  gen.  body 


Basis  pedunculi 


Dorsal  tegmental  decussation 

Ventral  tegmental  decussation 


Red  nucleus 
Oculomotor  nerve 


Substantia  nigra 


Fig.  116. — Section  through  the  mesencephalon  of  a  child  at  the  level  of  the  superior  colliculus. 

Pal-W'eigert  method.      (X  4.) 


medial  longitudinal  fasciculus  lies  immediately  ventral  to  the  nucleus  of  the 
trochlear  nerve  (Fig.  114).  In  the  pons  the  nucleus  of  the  abducens  nerve  is 
placed  on  its  dorsolateral  border.  The  close  relation  of  this  fascicle  to  the  nuclei 
for  the  motor  nerves  of  the  eye  is  of  considerable  significance,  since  according 
to  the  law  of  neurobiotaxis  (p.  179)  it  is  an  expression  of  the  fact  that  the  majority 
of  the  afferent  fibers  to  these  nuclei  come  from  this  fascicle.  This  bundle  of 
fibers,  the  composition  of  which  is  discussed  on  pages  152  and  329,  is  a  chief 
factor  in  the  reflex  control  of  the  movements  of  the  eyes,  and  especially  in  the 
coordination  of  these  movements  with  those  of  the  head  and  neck. 


nil.    IMI  l:\.\I.    STRUCTURE    OS    CHE    Ml  51  W(  I  I'll 

The  Lemnisci. — In  sections  through  the  rostral  border  of  the  pons  the  two 
temmsci  form  a  broad  curved  band  in  the  ventral  and  lateral  portions  of  the 
tegmentum.  The  fibers  of  the  lateral  lemniscus  are  cut  obliquely,  indicating 
that  they  have-  begun  to  turn  dorsally  toward  the  inferior  colliculus  (Fig.  112). 
()n  entering  the  midbrain  this  lateral  portion  of  the  fillet  separates  from  the 
medial  lemniscus  and  run-  toward  the  corpora  quadrigemina,  where  it  forms  a 
capsule  for  the  nucleus  of  the  inferior  colliculus  (Fig.  114).  Some  of  these 
fibers  are  prolonged  beyond  the  nucleus  and  decussate  with  similar  fibers  from 
the  opposite  side.  A  large  proportion  of  the  fibers  of  the  lateral  lemniscus  end 
in  the  inferior  colliculus.  but  other-  form  the  inferior  quadrigeminal  brachium 

Fig.  114).  through  which  they  reach  the  medial  geniculate  body    I  igs.  116,  134 
In  the  mesencephalon  the  lateral  lemniscus,  which,  it  will  be  remembered,  is  the 
central  auditory  tract  from  the  cochear  nuclei,  is  joined  by  the  fiber-  of  the 
spinotectal  tract;  and  these  run  with  it  to  the  corpora  quadrigemina. 

The  medial  lemniscus,  or  bulbothalamic  tract  from  the  gracile  and  cuneate 
nuclei  of  the  opposite  side,  is  continued  through  the  tegmentum  of  the  mesen- 
cephalon to  end  in  the  lateral  nucleus  of  the  thalamus  (Fig.  235).  Incorporated 
with  it  in  this  upper  part  of  its  course  are  the  fibers  of  the  spinothalamic  tract 
and  a  portion  of  the  central  sensory  tract  of  the  trigeminal  nerve  (Figs.  132.  234). 
In  the  caudal  part  of  the  mesencephalon  this  broad  band  of  longitudinal  fiber.-, 
occupies  the  ventrolateral  portion  of  the  tegmentum  (Fig.  1 14) ;  but  at  the  level 
of  the  superior  colliculus  it  has  been  displaced  dorsolaterally  by  the  red  nucleus. 
Here  it  lies  not  far  from  the  medial  geniculate  body  and  inferior  quadrigeminal 
brachium  (Fig.  116). 

The  Central  Gray  Stratum. — The  cerebral  aqueduct  is  lined  by  ependymal 
epithelium  and  surrounded  by  a  thick  layer  of  gray  matter,  the  central  gray 
stratum,  which,  because  of  its  paucity  in  myelinated  fibers,  is  nearly  colorless  in 
Weigert  preparations.  This  layer  is  continuous  with  the  gray  matter  surround- 
ing the  third  ventricle,  on  the  one  hand,  and  with  that  covering  the  rhomboid 
fossa  on  the  other.  Numerous  nerve-cells  of  various  size  and  -hape  are 
tered  through  this  central  gray  substance;  and.  in  addition,  there  are  three 
compact  groups  of  cells,  which  are  the  nuclei  of  the  oculomotor  and  trochlear 
nerves  and  of  the  mesencephalic  root  of  the  trigeminus. 

The  nucleus  of  the  trochlear  nerve  contains  the  cells  of  origin  of  the  motor 
fibers  for  the  superior  oblique  muscle  of  the  eye.  It  is  a  small  oval  mass  -ituated 
in  the  ventral  part  of  the  central  gray  Stratum  at  the  level  of  the  inferior  collic- 
ulus (Fig.  114).     The  fibers  of  the  trochlear  nerve  emerge  from  the  dorsolateral 


164  THE    NERVOUS    SYSTEM 

aspect  of  this  nucleus,  curve  dorsally  around  the  central  gray  matter,  and  decus- 
sate in  the  anterior  medullary  velum  (Fig.  112). 

The  nucleus  of  the  oculomotor  nerve  is  composed  of  the  cells  of  origin  of 
the  motor  fibers  for  all  of  the  ocular  muscles  except  the  superior  oblique  and 
lateral  rectus.  It  lies  in  the  ventral  part  of  the  central  gray  substance  beneath 
the  superior  colliculus  (Fig.  116).  This  nucleus,  a  part  of  which  occupies  a 
median  position  and  supplies  fibers  to  the  nerves  of  both  sides,  is  6  or  7  mm. 
long  and  extends  from  a  little  beyond  the  rostral  limit  of  the  mesencephalon  to 
the  nucleus  of  the  trochlear  nerve,  from  which  it  is  not  sharply  separated.  From 
the  nucleus  the  fibers  of  the  oculomotor  nerve  stream  forward  through  the 
tegmentum  and  red  nucleus.  They  emerge  through  the  oculomotor  sulcus  along 
the  ventromedial  surface  of  the  basis  pedunculi. 

The  interpeduncular  ganglion  is  a  median  collection  of  nerve-cells  in  the 
posterior  perforated  substance  situated  between  the  two  cerebral  peduncles  near 
the  border  of  the  pons  (Fig.  114).  It  receives  fibers  from  the  habenular  nucleus 
of  the  epithalamus  by  way  of  the  fasciculus  retroflexus  of  Meynert;  and  from 
it  spring  fibers  that  run  to  the  dorsal  nucleus  of  the  tegmentum  (Tig.  211). 

The  substantia  nigra  is  a  broad  thick  plate  of  pigmented  gray  matter,  which 
separates  the  basis  pedunculi  from  the  tegmentum  and  extends  from  the  border 
of  the  pons  throughout  the  length  of  the  mesencephalon  into  the  hypothalamus. 
In  transverse  section  it  presents  a  semilunar  outline.  Its  medial  border  is  super- 
ficial in  the  oculomotor  sulcus  and  is  thicker  than  the  lateral  border,  which 
reaches  the  lateral  sulcus  of  the  mesencephalon.  Its  constituent  nerve-cells, 
irregular  in  shape  and  deeply  pigmented,  send  their  axons  into  the  tegmentum. 
But  we  are  still  ignorant  as  to  the  destination  these  may  have;  and  the  func- 
tion of  the  substantia  nigra  is  equally  obscure.  It  receives  collaterals  from  the 
corticifugal  fibers  of  the  basis  pedunculi.  Furthermore,  there  terminates  within 
it  a  bundle,  consisting  of  both  direct  and  crossed  fibers  from  the  corpus  striatum, 
the  strionigral  tract  (Fig.  117). 

The  basis  pedunculi  is  a  broad  compact  strand,  crescentic  in  transverse  sec- 
tion, which  consists  of  longitudinal  fibers  of  cortical  origin.  These  are  con- 
tinued from  the  internal  capsule  into  the  longitudinal  bundles  of  the  pons 
through  the  basis  pedunculi.  It  consists  of  four  tracts.  The  medial  and  lat- 
eral fifths  are  occupied  by  fibers  which  terminate  in  the  nuclei  pontis.  Those 
of  the  medial  one-fifth  arise  from  the  cortex  of  the  frontal  lobe  of  the  cerebral 
hemisphere  and  constitute  the  frontopontine  tract.  Other  fibers,  arising  from 
the  temporal  lobe,  form  the  temporopontine  tract  and  occupy  the  lateral  one- 


THE    l.MKKNAI.    STRUCTUR]     "I      Mil      MESENCEPHALON 


[65 


fifth  of  the  basis  pedunculi.  The  intermediate  portion,  approximately  three- 
fifths,  is  formed  by  the  corticospinal  tract,  the  fibers  of  which  after  giving  off 
collaterals  to  the  nuclei  pontis  are  continued  into  the  pyramids  of  the  medulla 
oblongata  and  thence  into  the  spinal  cord.  Many  of  the  fibers  of  the  cortico- 
bulbar  trad  are  intermingled  with  the  more  medially  placed  corticospinal  fibers; 
but  even  at  this  level  two  large  Fascicles  destined  for  the  nuclei  of  the  cranial 
nerves  have  separated  from  the  main  strand  of  motor  fibers  (Dejerine,  1914). 
These  have  been  called  the  medial  and  lateral  corticobulbar  tracts  (Figs.  1.06, 
117). 

The  Corpora  Quadrigemina. — The  rostral  portion  of  the  midbrain  roof  or 
latum  mesencephali  is  in  all  vertebrates  an  end-station  for  the  optic  tracts.  In 
the  lower  vertebrates  there  are  but  two  elevations  in  the  roof,  the  optic  lobes  or 
corpora  bigemina,  and  these,  which  correspond  in  a  general  way  to  the  superior 


Temporopontine  tract 
Tr.  corticobulbaris  tat. 

Strionigral  tract 
Corticospinal  tract 


Frontopontine  tract  Tr.  corticobulbaris  med. 

Fig.  117. — Diagram  of  the  basis  pedunculi. 

colliculi,  are  visual  centers  (Fig.  13).  In  mammals  the  development  of  a  spir- 
ally wound  cochlea  is  associated  with  the  appearance  of  two  additional  eleva- 
tions, the  inferior  colliculi,  within  which  many  of  the  fibers  of  the  central  audi- 
tory path  terminate.  The  entire  tectum  receives  fibers  from  the  spinal  cord 
and  medulla  oblongata  and  sends  other  fibers  back  to  them;  it  also  receives  fibers 
from  the  cerebral  cortex.  It  contains  important  reflex  centers,  those  in  the 
superior  colliculus  being  dominated  by  visual,  those  in  the  inferior  colliculus 
by  auditory,  impulses. 

The  inferior  colliculi  or  inferior  quadrigeminal  bodies  each  contain,  in  addi- 
tion to  the  laminated  gray  matter  of  the  tectum,  a  large  gray  mass,  oval  in 
transverse  section,  and  known  as  the  nucleus  of  the  inferior  colliculus  (Fig.  114). 


1 66 


THE   NERVOUS    SYSTEM 


The  lateral  lemniscus  has  been  traced  to  this  nucleus,  and  while  some  of  the 
libers  plunge  directly  into  it,  others  sweep  around  it  to  form  a  capsule,  within 
which  it  is  enclosed.  The  majority  of  these  fibers  ultimately  end  in  this  nu- 
cleus, but  some  pass  beyond  it,  reach  the  median  plane,  and  decussate  with  sim- 
ilar fibers  from  the  opposite  side  (Fig.  118).  The  ramifications  of  fibers  from  the 
lateral  lemniscus  form  an  intricate  interlacement  within  the  nucleus,  and 
throughout  this  network  are  scattered  many  nerve-cells  of  various  shapes  and 


Fig.  118. — Semidiagrammatic  section  through  the  inferior  colliculus  of  the  mouse:  A,  Nucleus 
of  inferior  colliculus;  B,  gray  matter  of  the  lamina  quadrigemina;  C,  inferior  quadrigeminal  bra- 
chium;  D,  central  gray  substance;  K,  decussation  of  the  brachium  conjunctivum;  a,  b,  c,  d,  fibers 
of  the  lateral  lemnisus.     Golgi  method.     (Cajal.) 

sizes.  On  the  medial  side  of  this  circumscribed  nuclear  mass  we  find  some  of 
the  laminated  gray  matter  of  the  tectum,  within  which  are  embedded  large  mul- 
tipolar cells  with  axons  directed  ventrally  in  the  stratum  profundum.  These 
partially  encircle  the  central  gray  matter  and  after  undergoing  a  partial  decus- 
sation enter  the  tectobulbar  and  tectospinal  tracts. 

The  inferior  quadrigeminal  brachium  begins  on  the  lateral  side  of  the  nucleus 
of  the  inferior  colliculus  and  consists  of  fibers  from  the  lateral  lemniscus  which 


THE    1XTKKNAL    STRIX'TI'KK    OF    Till;    MESENCEPHALON  [67 

run  to  and  terminate  within  the  medial  geniculate  body  (Figs.  114.  116  The 
fibers  of  the  lateral  Lemniscus  carry  auditory  impulses  from  the  terminal  uuclei 
of  the  cochlear  nerve.  Some  of  these  terminate  in  the  inferior  colliculus  and 
are  concerned  with  reflexes  in  response  to  sound.  Other  fibers,  some  of  which 
are  branches  of  those  to  the  inferior  colliculus,  run  to  the  medial  genii  ulate 
body,  from  which  the  impulses  that  they  carry  are  relayed  to  the  cerebral  cor 
tex.  The  inferior  quadrigeminal  brachium  also  contains  fibers  of  i  ortii  al  origin, 
chiefly  from  the  temporal  lobe,  which  end  within  the  inferior  colliculus  (Beevor 
and  Horsley.  1902). 

The  superior  colliculi,  or  superior  quadrigeminal  bodies,  are  composed  of 
laminated  gray  matter.  Each  consists  of  four  superimposed,  dorsally  convex 
layers  (Fig.  116).  The  most  superficial  of  these  is  a  thin  lamina  with  many 
transversely  coursing  nerve-fibers,  the  stratum  zonale.  The  second  layer  is  much 
thicker,  contains  few  myelinated  fibers,  and  is  knowrn  as  the  stratum  grtseum. 
The  third  and  fourth  layers,  stratum  opticam  and  stratum  lemnisci,  are  rich  in 
myelinated  fibers.  The  majority  of  the  afferent  fibers  of  the  superior  colliculus 
come  from  the  optic  tract  by  way  of  the  superior  quadrigeminal  brachium  and 
enter  the  stratum  opticum.  Many  of  these  end  in  the  superimposed  stratum 
griseum.  The  superior  colliculus  also  receives  fibers  from  the.  cerebral  cortex 
and  from  the  spinotectal  tract. 

It  has  been  generally  supposed  that  the  fibers  of  the  stratum  zonale  come  from  the 
optic  tract,  but  according  to  Cajal  (1911)  this  cannot  be  the  case,  since  they  remain  intact 
in  animals  which  have  been  operated  on  in  such  a  way  as  to  produce  degeneration  of  the  optic 
fibers.  According  to  him  it  is  also  probable  that  the  fibers  from  the  cerebral  cortex,  which 
reach  the  colliculus  by  way  of  the  superior  quadrigeminal  brachium,  end  in  the  stratum 
lemnisci.  The  fibers  of  the  spinotectal  tract  run  with  the  lateral  lemniscus  in  the  upper  part 
of  its  course  and  enter  the  superior  colliculus  by  way  of  the  stratum  profundum. 

The  tectobulbar  and  tectospinal  tracts  have  their  origin  within  the  tectum  of 
the  mesencephalon,  more  of  the  fibers  coming  from  the  superior  than  from  the 
inferior  colliculi.  These  fibers,  arising  from  cells  in  more  superficial  layers,  are 
assembled  in  the  stratum  profundum  and  sweep  ventrally  in  broad  curves  around 
the  central  gray  substance  (Figs.  116,  118).  The  majority  of  the  fibers,  after 
crossing  the  median  plane  in  the  dorsal  tegmental  decussation,  run  in  a  caudal 
direction  just  ventral  to  the  medial  longitudinal  bundle  in  the  tectospinal  tract. 
They  give  off  collaterals  to  the  reticular  formation  and  the  red  nucleus.  But 
some  of  them,  instead  of  taking  part  in  this  decussation,  leave  the  mesencephalon 
by  way  of  the  lateral  lemniscus  of  the  same  side,  constituting  the  lateral  tecto- 
bulbar and  tectospinal  tracts  (Cajal.  1911;  Edinger,  1911). 


CHAPTER  XII 

THE  CRANIAL  NERVES  AND  THEIR  NUCLEI 

The  cranial  nerves  contain,  in  addition  to  the  general  somatic  and  visceral 
components,  which  were  encountered  in  the  study  of  the  spinal  nerves,  also 
other  functional  groups  of  fibers  of  more  restricted  distribution  and  specialized 
function.  These  special  somatic  and  visceral  components  supply  the  organs  of 
special  sense  and  the  visceral  musculature,  derived  from  the  branchial  arches, 
which  differs  from  other  visceral  musculature  in  that  it  is  striated.  The  fibers 
which  supply  this  special  musculature  are  designated  as  special  visceral  efferent 
fibers.  The  eye  and  ear,  being  special  somatic  sense  organs,  are  supplied  by 
special  somatic  afferent  fibers.  The  olfactory  mucous  membrane  and  the  taste 
buds  are  special  visceral  sense  organs  and  are  supplied  by  special  visceral  af- 
ferent fibers. 

From  what  has  been  said  it  will  be  evident  that  there  are  seven  distinct 
functional  components  in  the  cranial  nerves,  namely:  somatic  efferent,  general 
somatic  afferent,  special  somatic  afferent,  general  visceral  efferent,  special  vis- 
ceral efferent,  general  visceral  afferent,  and  special  visceral  afferent  components 
(Figs.  119,  120).  No  single  nerve  contains  all  seven  types  of  fibers  and  the 
individual  cranial  nerves  vary  greatly  in  their  functional  composition.  On 
entering  the  brain  a  nerve  breaks  up  into  its  several  components,  which  separate 
from  each  other  and  pass  to  their  respective  nuclei,  enumerated  below.  These 
nuclei  may  be  widely  separated  in  the  brain  stem.  Fibers  having  the  same  func- 
tion tend  to  be  associated  together  within  the  brain  irrespective  of  the  nerves 
to  which  they  belong.  For  example,  all  the  visceral  afferent  fibers  of  the  facial, 
glossopharyngeal,  and  vagus  nerves  are  grouped  in  the  tractus  solitarius  (Fig. 
120,  yellow).  The  nerve-cells,  with  which  the  fibers  of  the  several  functional 
varieties  are  associated  within  the  brain  stem,  are  arranged  in  longitudinal 
nuclear  columns.  The  analysis  of  the  cranial  nerves  into  their  functional  com- 
ponents has  involved  a  great  amount  of  labor  which  has  been  carried  through 
for  the  most  part  by  American  investigators.  Among  those  who  have  made 
important  contributions  to  this  subject  may  be  mentioned  the  following:  Gas- 
kell  (1886),  Strong  (1895),  Herrick  (1899),  Johnston  (1901),  Coghill  (1902), 
Norris  (1908),  and  Willard  (1915). 

1 68 


Special  somatic  afferent,, 
nucleus 

Gent  ral  somatii  afferentm.[. \ r*^ 

nucleus 

Alar  lamina" 

Visceral  afferent  nucleus' 

Genera!  visceral  efferent _ 

nucleus 

Special  visceral  efferent- 

nucleus 

Basal  lamina' 

Somatic  efferent  nucleus' 


Somatic  muscle 
Sympathetic  ganglion 

Visceral  mucous  membrane 
Smooth  muscle 


*N  Sensory  ganglion 
Branchial  muscle 


Fig.  119. 


Sensory  nucleus  N.  I 
Nucleus  of  abducens  nerve\   \ 

Facial  nerve*  \    ^ 
Vestibular  nuclei       \    \ 
\      \    \ 


Vestibular  ganglion 

and  nerve  < 


Bulbar  rootlet  of  accessory  nerve 
Spinal  root  of  accessory  nerve-' 
Nucleus  ambiguus-' 
Traclus  solitarius 


Nucleus  of  Edinger-Westphal 
Nucleus  of  oculomotor  nerve 

Nucleus  of  trochlear  nerve 
Mesencephalic  nucleus  N.  V 

Trigeminal  nerve 
and  semilunar 
ganglion 
.  Spinal  trad  and 
nucleus  X.  V 

.-  Cm  hlear  nuclei 

■  Spiral  ganglion 
and  cochlear 
nerve 

.Glossopharyn- 
geal nerve 
■  Vagus  nerve 

Nuc.  salivatorius  superior 
Xuc.  salivatorius  inferior 
Dorsal  motor  nucleus  N.  X 


Cervical  spinal  nerve 


Fig.  120. 
Figs.  119  and  120.— Diagrams  showing  the  origin,  course,  and  termination  of  the  functional 
components  of  the  cranial  nerves.  Somatic  afferent  and  efferent,  red;  visceral  afferent,  yellow; 
general  visceral  efferent,  black;  special  visceral  efferent,  blue.  Fig.  1 19  shows  the  locations  of  the 
several  functional  cell  columns  in  a  section  through  the  medulla  oblongata  of  a  human  embryo  and 
the  peripheral  terminations  of  the  several  varieties  of  fibers.  Fig.  120,  dorsal  view  of  the  human 
brain  stem,  showing  the  location  of  the  nuclei  and  the  intramedullary  course  of  the  fibers  of  the 
cranial  nerves. 


170  THE    NERVOUS    SYSTEM 

Longitudinal  Nuclear  Columns. — In  a  previous  chapter  we  learned  that  at 
an  early  stage  in  its  development  the  lateral  wall  of  the  neural  tube  consists  of  a 
dorsal  or  alar  and  a  ventral  or  basal  plate,  separated  by  a  groove,  the  sulcus 
li nutans  (Fig.  119).  The  sensory  nuclei  of  the  cranial  nerves  develop  within  the 
alar  plate  and  the  motor  nuclei  within  the  basal  plate.  In  the  rhombencephalon 
both  plates  come  to  He  in  the  floor  of  the  fourth  ventricle,  the  alar  occupying 
the  more  lateral  position.  And,  in  spite  of  the  changes  of  position  which  occur 
during  development,  the  sensory  nuclei  retain,  on  the  whole,  a  lateral,  and  the 
motor  nuclei  a  more  medial,  location.  From  the  basal  plate  there  differentiate 
a  somatic  and  a  visceral  column  of  efferent  nuclei,  and  from  the  alar  plate  a 
visceral  and  a  somatic  column  of  afferent  nuclei. 

The  somatic  efferent  column  includes  the  nuclei  of  those  motor  nerves  which 
supply  the  striated  musculature  derived  from  the  myotomes,  i.e.,  the  extrinsic 
muscles  of  the  eye  and  the  musculature  of  the  tongue  (Figs.  119-121). 

The  visceral  efferent  column  undergoes  subdivision  into:  (1)  a  ventrolateral 
column  of  nuclei,  from  which  arise  the  special  visceral  efferent  fibers  to  the  striated 
visceral  or  branchial  musculature,  and  which  includes  the  nucleus  ambiguus  and 
the  motor  nuclei  of  the  fifth  and  seventh  nerves;  and  (2)  a  more  dorsally  placed 
group  for  the  innervation  of  involuntary  musculature  and  glandular  tissue,  of 
which  the  dorsal  motor  nucleus  of  the  vagus  is  the  chief  example.  The  former 
may  be  called  the  special  visceral  efferent  and  the  latter  the  general  visceral  ef- 
ferent column. 

The  visceral  afferent  column  is  represented  by  the  nucleus  of  the  tractus 
solitarius,  within  which  end  the  afferent  fibers  from  the  visceral  mucous  membrane 
and  the  taste  buds,  i.  e.,  both  the  general  and  special  visceral  afferent  fibers. 
The  somatic  afferent  column  splits  into  two:  a  general  somatic  afferent  column. 
within  which  terminate  the  sensory  fibers  from  the  skin;  and  a  special  somatic 
group  of  nuclei  for  the  reception  of  the  fibers  of  the  acoustic  nerve  and,  in  aquatic 
vertebrates,  of  the  lateral  line  nerves  also. 

THE  SOMATIC  EFFERENT  COLUMN 

As  can  be  seen  by  reference  to  Figs.  101.  108,  114.  and  116  the  nuclei  of  the 
hypoglossal,  abducens,  trochlear,  and  oculomotor  nerves  are  arranged  in  linear 
order  in  the  central  gray  matter  near  the  median  plane.  They  represent  the 
continuation  into  the  medulla  oblongata  of  the  large  cells  of  the  anterior  column 
of  the  spinal  cord.  The  cells  of  these  nuclei  are  large  and  multipolar  with 
well-developed   Xissl   bodies   (Fig.   126).     From    them   arise   large  myelinated 


THE    CRANIAL   NERVES    AND    Til  KIR    NUCLEI 


I7I 


fibers,  which  innervate  the  striated  musculature  derived  from  the  myotomes. 
This  group  of  nuclei  is  indicated  in  red  in  Fig.  120  and  by  small  circles  in  Figs. 
121  and  122. 

The  nucleus  of  the  oculomotor  nerve  is  an  elongated  mass  of  cells  in  the  ceil 
tral  gray  matter  ventral  to  the  cerebral  aqueduct  at  the  level  of  the  superi<  r 
colliculus  (Figs.   121,   122).     Even  a  superficial  examination  shows  that  it  is 
divided  into  a  lateral  paired  and  a  medial  unpaired  portion  (Tig.   116).     The 


Nuc.  Ill  E-W. 
Nuc.  Ill  [at. 


Trigonum 
hypoglossi 
Nuc.  spinalis  V 


Nuc.  com. 
Cajal 


Fig.  121. — Dorsal  view  of  the  human  brain  stem  with  the  positions  of  the  cranial  nerve  nuclei 
projected  upon  the  surface.  Sensory  nuclei  on  the  right  side,  motor  nuclei  on  the  left.  Circles 
indicate  somatic  efferent  nuclei;  small  dots,  general  visceral  efferent  nuclei;  large  dots,  special 
visceral  efferent  nuclei;  horizontal  lines,  general  somatic  sensory  nuclei;  cross-hatching,  visceral 
sensory  nuclei;  stipple,  special  somatic  sensory  nuclei.      (Herrick.) 


lateral  groups  of  cells  spreads  out  upon  the  surface  of  the  medial  longitudinal 
bundle,  extends  throughout  the  entire  length  of  the  nucleus,  and  may  be  divided 
into  ventral  and  dorsal  portions  (Fig.  123).  The  medial  group  of  cells  is  placed 
exactly  in  the  median  plane  and  is  found  only  in  the  rostral  half  of  the  nucleus. 
Dorsolateral  from  this  median  group,  and  restricted  to  the  most  rostral  part  of 
the  nucleus,  is  a  collection  of  small  cells  which  form  the  nucleus  of  Edinger- 
Westphal.     This  is  a  visceromotor  nucleus  and  will  be  considered  elsewhere. 


172 


THE    NERVOUS    SYSTEM 


The  fibers  from  the  medial  nucleus  enter  both  right  and  left  nerves.  Some 
from  the  caudal  portion  of  the  dorsal  division  of  the  lateral  nucleus  cross  the 
median  plane.  The  others  remain  uncrossed.  After  sweeping  in  broad  curves 
through  the  tegmentum  and  red  nucleus  the  fibers  emerge  through  the  oculo- 
motor sulcus.  All  of  the  extrinsic  muscles  of  the  eye  except  the  lateral  rectus 
and  superior  oblique  are  supplied  by  the  medial  and  lateral  groups  of  cells  just 
described. 

t ,  Nucleus  of  Edinger-W est phal 
,. Nucleus  of  oculomotor  nerve 
)  . — -Corpora  quadrigcmina 
Cerebral  aqueduct 
Nucleus  of  trochlear  nerve 
Trochlear  nerve 

Anterior  medullary  velum 

..-■' Motor  nucleus  N.  V 
_,  -  'Nucleus  of  facial  nerve 
Fourth  ventricle 
Nucleus  of  abducens  nerve 

Nuc.  salivatorius  superior 

Nuc.  salivatorius  inferior 
-  Nucleus  of  hypoglossal  nerve 

—  Dorsal  motor  nucleus  N.  X 
^--Central  canal 

—  Nucleus  ambiguus 


Mesencephalon 
Oculomotor  nerve 

Pons-  • 

Portio  minor  N.  V- 

Facial  nerve  ■ 
Abducens  nerve - 
Medulla  oblongata 


Fig.  122. — Motor  nuclei  of  the  cranial  nerves  projected  on  a  median  sagittal  section  of  the 
human  brain  stem.  Circles  indicate  somatic  efferent  nuclei;  small  dots,  general  visceral  efferent 
nuclei;  large  dots,  special  visceral  efferent  nuclei. 


As  one  might  expect  from  the  fact  that  the  oculomotor  nerve  supplies  several  distinct 
muscles,  its  nucleus  seems  to  be  made  up  of  a  number  of  more  or  less  distinct  groups  of  cells; 
but  the  efforts  to  locate  subordinate  nuclei  have  given  rise  to  contradictory  results.  The 
most  significant  work  in  this  field  has  been  done  by  Bernheimer  (1904),  who  extirpated  in- 
dividual eye  muscles  in  monkeys  and  studied  the  resultant  changes  in  the  cells  of  the  oculo- 
motor nuclei.  According  to  him,  the  various  muscles  are  supplied  by  the  lateral  nucleus  in 
the  following  order,  beginning  at  the  rostral  end:  levator  palpebral  superioris,  rectus  supe- 
rior, rectus  medialis,  obliquus  inferior,  and  rectus  inferior.  Bernheimer  says  that  the  fibers 
for  the  rectus  inferior  are  entirely  crossed,  those  for  the  obliquus  inferior  are  in  greater  part 
crossed,  those  for  the  rectus  medialis  for  the  most  part  uncrossed,  those  for  the  rectus  superior 
and  levator  palpebral  superioris  entirely  uncrossed. 


IHE    (RAMAI.    NERVES    AND    I  III  Ik    M  I  l.u 


173 


The  nucleus  of  the  trochlear  nerve  has  already  been  located  in  the  central 
pray  matter  ventral  to  the  cerebral  aqueduct  at  the  level  of  the  inferior  collic- 
uluSj  close  to  the  caudal  extremity  of  the  oculomotor  nucleus  Figs.  111.  121, 
122).  The  fibers  of  the  trochlear  nerve  emerge  from  the  dorsal  and  lateral 
aspects  of  this  nucleus,  and.  encircling  the  central  gray  matter  along  an  angular 
course  which  carries  them  also  caudally,  enter  the  anterior  medullary  velum, 
decussate  within  it.  and  make  their  exit  from  its  dorsal  surface  1  Fig.  112).  The) 
supply  the  superior  oblique  muscle. 

The  nucleus  of  the  abducens  nerve  was  encountered  in  the  dorsal  portion 
of  the  pons  as  a  spheric  gray  mass,  which  with  the  genu  of  the  facial  nerve  forms 
the  facial  colliculus  of  the  rhomboid  fossa  (Figs. 
108,  121,  122).  The  fibers  of  the  abducens  nerve 
leave  the  nucleus  chiefly  on  its  dorsal  and  medial 
surfaces  and  become  assembled  into  several  root 
bundles,  which  are  directed  ventrally  toward  their 
exit  from  the  lower  border  of  the  pons  near  the 
pyramid  of  the  medulla  oblongata.  It  supplies 
the  lateral  rectus  muscle. 

The  axons,  which  ramify  within  the  three  nuclei 
for  the  motor  nerves  of  the  eye,  are  derived  from 
many  sources.  The  most  important  of  these 
sources  are  the  corticobulbar  tract,  the  medial  lon- 
gitudinal bundle,  and  the  tectospinal  tract.  The 
nucleus  of  the  abducens  receives  fibers  also  from 
the  central  auditory  apparatus  through  the  pe- 
duncle of  the  superior  olive.  These  various  fibers 
provide  for  voluntary  movements  of  the  eyes,  and 

for  reflex  ocular  movements  in  response  to  vestibular,  visual,  and  auditory 
impulses.  The  nuclei  probably-  also  receive  branches  from  the  central  sensory 
path  of  the  fifth  nerve. 

The  nucleus  of  the  hypoglossal  nerve  is  a  slender  cylindric  mass  of  gray 
matter  nearly  2  cm.  in  length,  extending  from  the  level  of  the  fovea  inferior  to 
that  of  the  decussation  of  the  pyramids.  We  have  already  identified  it  in  both 
the  open  and  the  closed  portions  of  the  medulla  oblongata  (Figs.  99,  103).  In  the 
floor  of  the  fourth  ventricle  it  lies  beneath  the  trigonum  hypoglossi.  while  more 
caudally  it  lies  ventral  to  the  central  canal  (Figs.  121,  122).     The  root  fibers 


Fig.  123. — Diagram  of  the 
nuclei  of  the  oculomotor  nerve: 
M,  Median  nucleus;  £.11*.,  nu- 
cleus of  Edinger-\Y estphal;  V.L., 
D.L.,  ventral  and  dorsal  portions 
of  the  lateral  nucleus.  (Ober- 
steiner.) 


174  THE    NERVOUS    SYSTEM 

are  assembled  into  bundles  which  run  ventrally  toward  their  exit  along  the 
lateral  border  of  the  pyramid. 

A  conspicuous  plexus  of  myelinated  fibers  gives  the  hypoglossal  nucleus  a 
characteristic  appearance  in  Weigert  preparations.  Fibers  from  many  sources 
reach  the  nucleus  and  ramify  within  it.  These  include  some  from  the  cortico- 
bulbar  tract  and  others  from  the  sensory  nuclei  of  the  fifth  nerve  and  from  the 
nucleus  of  the  tractus  solitarius.  The  part  which  such  fibers  may  play  in  reflex 
movements  of  the  tongue  is  illustrated  in  Fig.  92. 

THE  SPECIAL  VISCERAL  EFFERENT  COLUMN 

The  special  visceral  efferent  column  of  nuclei  contains  the  cells  of  origin  of 
the  motor  fibers  for  the  striated  musculature  derived  from  the  branchial  arches, 
as  distinguished  from  the  general  skeletal  musculature  that  develops  from 
the  myotomes.  The  branchial  musculature  includes  the  following  groups  of 
muscles:  the  muscles  of  mastication,  derived  from  the  mesoderm  of  the  first 
branchial  arch  and  innervated  by  the  trigeminal  nerve;  the  muscles  of  expression, 
derived  from  the  second  or  hyoid  arch  and  innervated  by  the  facial  nerve;  the 
musculature  of  the  pharnyx  and  larynx,  derived  from  the  third  and  fourth  arches 
and  innervated  by  the  glossopharyngeal,  vagus,  and  accessory  nerves;  and  prob- 
ably also  the  sternocleidomastoid  and  trapezius  muscles,  innervated  through  the 
spinal  root  of  the  accessory  nerve.  Some  authors  prefer  to  call  this  column, 
which  includes  the  motor  nuclei  of  the  fifth  and  seventh  nerves  and  the  nucleus 
ambiguus,  the  lateral  somatic  column,  because  the  cells  in  these  nuclei  and  the 
fibers  which  arise  from  them  possess  the  characteristics  of  somatic  motor  cells 
and  fibers  (Malone,  1913).  The  nuclei  are  composed  of  large  multipolar  cells 
with  well-developed  Nissl  bodies.  These  cells  give  origin  to  large  myelinated 
fibers  which  run  through  the  corresponding  nerve  and  terminate  in  neuromus- 
cular endings  in  one  or  another  of  the  muscles  indicated  above. 

The  motor  nuclei  of  the  fifth  and  seventh  nerves  and  the  nucleus  ambiguus 
of  the  ninth,  tenth,  and  eleventh  nerves  form  a  broken  column  of  gray  matter, 
located  in  the  ventrolateral  part  of  the  reticular  formation  of  the  pons  and 
medulla  oblongata  some  distance  beneath  the  floor  of  the  fourth  ventricle  (Figs. 
121,  122).  The  cells  of  this  column  and  the  special  visceral  efferent  fibers  which 
arise  from  them  have  been  colored  blue  in  Figs.  119  and  120. 

The  motor  nucleus  of  the  trigeminal  nerve  lies  on  the  medial  side  of  the 
main  sensory  nucleus  of  that  nerve,  and  is  located  at  the  level  of  the  middle 
of  the  pons  in  the  lateral  part  of  the  reticular  formation  some  distance  from  the 


THK    CRANIAL    NKRVKS    AND    T1IKIK     M  I   II   I 


'75 


ventricular  floor  (Figs.  110,  121,  122).    The  fibers,  which  take  their  origin  here, 

arc  collected  in  the  motor  root  or  portio  minor  of  the  fifth  nerve  and  run  with  ii 
mandibular  division  to  the  muscles  of  mastication.  Within  the  nucleus  then- 
terminate  fibers  from  the  corticobulbar  tract  and  many  fibers,  chiefly  collaterals 
from  the  central  sensory  tract  of  the  trigeminal  nerve.  It  also  receives  collat- 
erals from  the  mesencephalic  root  of  the  trigeminal  and  from  other  sources 
(Fig.  131). 

The  motor  nucleus  of  the  facial  nerve  is  located  in  the  ventrolateral  part 
of  the  reticular  formation  of  the  pons  near  its  caudal  border  (Figs.  108,  121, 
122).  Its  constituent  cells  are  arranged  so  as  to  form  a  varying  number  of  sub- 
groups which  may  possibly  be  concerned  with  the  innervation  of  individual  facial 
muscles. 


Root  of  facial  nerve,  first  part 
Ibducens  nucleus 


Root  of  facial  nerve,  genu 


Root  of  facial  nerve,  second  part 
Facial  nucleus 


Nucleus  of  ahducens  nerve 

Root  filaments  of  ahducens  nerve 

Stalk  of  superior  olive 

Root  of  facial  nerve,  first  part 
Spinal  root  and  nuch  u    AT 
A  ucleus  of  facial  m 
Root  official  )i.. 
Superior  olive   [part 


A  bducens  nerve 


Fig.  124. — Diagram  of  the  root  of  the  facial  nerve,  shown  as  if  exposed  by  dissection  in  a  thick 

section  of  the  pons. 

From  the  dorsal  aspect  of  this  nucleus  there  emerge  a  large  number  of  fine 
bundles  of  libers,  directed  dorsomedially  through  the  reticular  formation.  These 
rather  widely  separated  bundles  constitute  the  first  part  of  the  root  of  the  facial 
nerve  (Fig.  124).  Beneath  the  floor  of  the  fourth  ventricle  the  fibers  turn  sharply 
rostrad  and  are  assembled  into  a  compact  strand  of  longitudinal  fibers,  often 
called  the  ascending  part  of  the  facial  nerve.  This  ascends  along  the  medial  side 
of  the  abducens  nucleus  dorsal  to  the  medial  longitudinal  bundle  for  a  consid- 
erable distance  (5  mm.).  The  nerve  then  turns  sharply  lateralward  over  the 
dorsal  surface  of  the  nucleus  of  the  abducens  nerve,  and  helps  to  form  the  eleva- 
tion in  the  rhomboid  fossa,  known  as  the  facial  colliculus.  This  bend  around 
the  abducens  nucleus,  including  the  ascending  part  of  the  facial  nerve,  is  known 


176 


TI1K    NERVOUS    SYSTEM 


as  the  genu.  The  second  part  of  the  root  of  the  facial  nerve  is  directed  ventro- 
laterally  and  at  the  same  time  somewhat  caudally,  passing  close  to  the  lateral 
side  of  its  own  nucleus,  to  make  its  exit  from  the  lateral  part  of  the  caudal 
border  of  the  pons  (Fig.  108). 

Fibers  from  many  sources  terminate  in  the  facial  nucleus  in  synaptic  rela- 
tion with  its  constituent  cells.  Those  from  the  corticobulbar  tract  place  the 
facial  muscles  under  voluntary  control.  Others  are  collaterals  from  the  sec- 
ondary sensory  paths  in  the  reticular  formation  and  are  concerned  with  bulbar 
reflexes.  Some  of  these  collaterals  are  given  off  by  fibers  arising  in  the  trapezoid 
body  and  carry  auditory  impulses.  Others  are  collaterals  of  fibers  arising  in 
the  nucleus  of  the  spinal  tract  of  the  fifth  nerve;  and  still  others  are  given  off  by 
ascending  sensory  fibers  from  the  spinal  cord  (Cajal,  1909). 


-  Vagus  nerve 

^" Jugular  foramen 

Internal  ramus  I  , 

c.  .       1  -  accessory  nerve 

hxtcrnal  ramus]  J 


Ibar  rootlets  of  accessory  nerve  ^.-zs 

; 

Foramen  magnum  -\ 

Spinal  root  of  accessory  nerve  

V 

/ 

/ 

/ 

/ 

/ 

/ 

/ 

A 

( 

1 

Fig.  125. — Diagram  of  the  roots  of  the  vagus  and  accessory  nerves. 


The  nucleus  ambiguus  is  a  long  slender  column  of  nerve-cells,  extending 
through  the  greater  part  of  the  length  of  the  medulla  oblongata  in  the  ventro- 
lateral part  of  the  reticular  formation  (Figs.  103,  121,  122).  Its  constituent 
cells  give  rise  to  the  special  visceral  efferent  fibers  that  run  through  the  glosso- 
pharyngeal, vagus,  and  accessory  nerves  to  supply  the  musculature  of  the 
pharynx  and  larynx.  It  reaches  from  the  border  of  the  pons  to  the  motor  de- 
cussation, but  is  most  evident  in  transverse  sections  through  the  caudal  part  of 
the  rhomboid  fossa.  Here  it  can  be  found  in  the  reticular  formation  ventral  to 
the  nucleus  of  the  spinal  root  of  the  trigeminal  nerve.  The  fibers  arising  from 
its  cells  are  at  first  directed  dorsally;  then  curving  laterally  and  ventrally  they 
join  the  root  bundles  of  the  ninth,  tenth,  and  eleventh  nerves  with  which  they 


Nil     .  R  \\l  \l.    \l  i:\  I  S    AND     nil  II'    \i  |  1.1  | 


i;; 


emerge  from  the  brain  (Fig.  105).    A  lew  of  tin-  fibei  the  median  plane 

ami  join  the  corresponding  root  bundles  of  tin-  opposite  side. 

The  accessory  nerve  consists  of  a  bulbar  am!  a  spinal  portion.  The  fibers  "i  the  spinal 
root  take  origin  from  a  linear  group  of  cells  in  the  lateral  pan  of  the  anterior  gray  column 
in  the  upper  cervical  segments  of  the  spinal  cord.  This  rool  ascends  along  ili<  side  of  the 
spinal  cord,  pasM-s  through  the  foramen  magnum,  and  is  joined  by  the  bulbar  rootlets  of  the 

sorj  (Fig.  I-'.m.  The  nerve  then  divides  into  an  internal  and  an  external  branch.  In 
the  latter  run  all  the  fibers  of  spinal  origin  and  these  are  distributed  to  the  trapeziu 
sternocleidomastoid  muscles.  If.  as  seems  probable,  these  muscles  are  derived  from  the 
branchial  arches  (Lewis,  1910),  the  fibers  which  supply  them  may  he  regarded  as  special 
viscera]  efferent  tihers;  and  the  spinal  nucleus  of  the  accessory  nerve  may  he  considered  as 
homologous  to  the  nucleus  ambiguus.  Tin-  bulbar  rootlets  of  the  accessory  nerve,  which  con- 
tain both  general  and  special  visceral  efferent  fibers,  form  a  well-denned  fascicle,  readily 
distinguished  from  the  spinal  portion  of  the  nerve,  which,  as  the  internal  ramus,  joins  the 
vagus  nerve  and  is  distributed  through  its  branches  (Fig.  120 — Chase  and  Ranson,  Y>\  I;. 

The  sensory  collaterals  which  arborize  among  the  cells  of  the  nucleus  am- 
biguus are  derived  from  the  central  tracts  of  the  trigeminal,  glossopharyngeal, 
and  vagus  nerves,  from  ascending  sensory  fibers  of  spinal  origin,  and  from  other 
longitudinal  fibers  in  the  reticular  formation.  Other  fibers  reach  this  nucleus 
from  the  corticobulbar  tract. 

THE  GENERAL  VISCERAL  EFFERENT  COLUMN 

The  general  visceral  efferent  column  of  nuclei  is  composed  of  the  cells  fri  im 
which  arise  the  efferent  libers  innervating  cardiac  and  smooth  muscle  and  glan- 


A 


■ 


A  B 

Fig.  126. — Two  types  of  motor  nerve-cells  from  medulla  oblongata  of  lemur:  .1,  Cells  of  the 
somatic  motor  type  from  the  hypoglossal  nucleus;  B,  cells  of  the  visceral  efferent  type  Irom  the 
rostral  part  of  the  dorsal  motor  nucleus  of  the  vagus.     Toluidin  blue  stain.      I  Mai 

dular  tissue.     The  cells  of  these  nuclei  are  of  small  or  medium  size  and  their 
Xissl  bodies  are  not  well  developed  (Fig.  126).    They  give  rise  to  the  general 


IJo  THE    NERVOUS    SYSTEM 

visceral  efferent  fibers  of  the  cranial  nerves.  These  are  small  myelinated  fibers, 
which  end  in  sympathetic  ganglia,  where  they  arborize  about  sympathetic 
cells,  the  axons  of  which  terminate  in  smooth  or  cardiac  muscle  or  in  glandular 
tissue.  The  neurons  of  this  series  are.  therefore,  characterized  by  the  fact  that 
the  impulses  which  they  transmit  must  be  relayed  by  neurons  of  a  second  order 
before  reaching  the  innervated  tissue  (Fig.  119).  This  group  of  nuclei  is  indi- 
cated by  black  in  Fig.  120  and  by  fine  stipple  in  Figs.  121  and  122. 

The  dorsal  motor  nucleus  of  the  vagus  nucleus  vagi  dorsalis  medialis)  has 
been  noted  in  the  transverse  sections  through  the  medulla  oblongata  (Figs.  99, 
103).  It  lies  along  the  dorsolateral  side  of  the  hypoglossal  nucleus,  subjacent 
to  the  ala  cinerea  of  the  rhomboid  fossa,  and  along  the  side  of  the  central  canal 
in  the  closed  part  of  the  medulla  oblongata.  The  general  visceral  efferent  fibers, 
which  arise  from  the  cells  in  this  nucleus,  leave  the  medulla  oblongata  through 
the  roots  of  the  vagus  and  accessory  nerves;  but  those  entering  the  accessory 
nerve  leave  that  nerve  by  its  internal  ramus  and  join  the  vagus  (Fig.  120). 
Hence  all  of  the  fibers  from  this  nucleus  are  distributed  through  the  branches  of 
the  vagus  to  the  vagal  sympathetic  plexuses  of  the  thorax  and  abdomen  for  the 
innervation  of  the  involuntary  musculature  of  the  heart,  respiratory  passages, 
esophagus,  stomach,  and  small  intestines  (Van  Gehuchten  and  Molhant.  1912), 
and  for  the  innervation  of  the  pancreas,  liver,  and  other  glands. 

There  are  relatively  few  sensory  collaterals  reaching  the  dorsal  motor  nucleus, 
and  these  come  in  large  part  from  sensory  fibers  of  the  second  order,  arising  in 
the  receptive  nuclei  of  the  trigeminal,  glossopharyngeal,  and  vagus  nerves. 

The  nucleus  salivatorius  is  located  in  the  reticular  formation,  some  distance 
from  the  floor  of  the  fourth  ventricle  at  the  junction  of  the  pons  and  medulla 
oblongata  near  the  caudal  end  of  the  facial  nucleus  and  the  rostral  end  of  the 
nucleus  ambiguus  (Figs.  121,  122).  The  more  caudal  portion,  or  nucleus  sal- 
ivatorius  inferior,  sends  general  visceral  efferent  fibers  by  way  of  the  glosso- 
pharyngeal nerve  to  the  otic  ganglion  for  the  innervation  of  the  parotid  gland. 
The  rostral  part,  or  nucleus  salivatorius  superior,  lies  dorsal  to  the  large  motor 
nucleus  of  the  facial  nerve,  to  which  nerve  it  sends  general  visceral  efferent 
fibers.  These  run  from  the  facial  nerve  through  the  chorda  tympani  to  the  sub- 
maxillary ganglion  for  the  innervation  of  the  submaxillary  and  sublingual  sal- 
ivary glands  'Kohnraamm.  1902.  1903.  1907:  Yagita,  1909;  Feiling,  1913). 

The  Edinger-Westphal  nucleus  i-  a  group  of  small  nerve-cells  located  in 
the  rostral  part  of  the  nucleus  of  the  oculomotor  nerve.  Here  it  is  placed 
dorsolateral  to  the  median  unpaired  portion  of  that  nucleus  (Figs.  121-123). 


Mil:    CRANIAL    NERVES    AND    mill'    NUCLEI 


179 


This  group  of  small  cells  gives  origin  to  the  general  visceral  efferent  fibers  of  the 
oculomotor  nerve  which  run  to  the  ciliary  ganglion  for  thi  innervation  of  the 
intrinsic  muscle  of  the  eye. 

Neurobiotaxis.  The  position  of  the  motor  nuclei  of  the  brain  stem  varii  -  greatly  in 
different  orders  of  vertebrates,  and  is  determined  by  the  source  of  tin-  principal  al 
impulses  which  reach  them.  The  perikarya  of  the  neurons  migrate  under  the  influent  c  of  an 
attraction,  which  has  been  called  neurobiotaxis,  in  the  direction  of  the  chief  fiber  tra<ts 
from  which  they  receive  impulses  (Ari6ns  (Cappers,  1(>14.  1()17;  Black,  1917).  "When  from 
different  places  stimuli  proceed  to  a  cell,  its  chief  dendrite  grows  out  and  its  <  el)  body  shifts  in 
the  direction  whence  the  majority  of  the  stimuli  proceed,"  while  the  axon  grows  in  the  op- 
posite direction  (Fig.  127).     The  nature  of  the  attractive  force  is  not  altogether  clear.     Kap- 


Axiscyli  ndcf 


B 

Fig.  127. — Diagram  to  illustrate  the  principle  of  neurobiotaxis.  The  axis-cylinder  grows  in 
the  direction  of  the  nervous  current,  indicated  by  the  arrow,  while  the  dendritic  outgrowth  and 
the  final  shifting  of  the  cell  body  occur  against  the  nervous  current:  A,  Dendrites  grown  out  to- 
ward the  center  of  stimulation;  B,  the  cell  body  has  shifted  toward  the  center  of  stimulation;  the 
axis-cylinder  is  consequently  elongated.      (Kappers.) 


pers  believes  that  it  is  a  galvanotropic  phenomenon,  on  the  basis  of  the  fact  that  the  stimu- 
lation center  is  electrically  negative,  i.  e.,  a  cathode  with  reference  to  the  surrounding  tissue. 

Numerous  instances  might  be  cited  of  the  action  of  this  taxis,  but  two  will  suffice.  It 
has  already  been  noted  that  the  eye-muscle  nuclei  receive  most  of  their  collaterals  from  the 
optic  and  vestibular  reflex  tracts;  and  these  appear  to  be  the  most  important  factors  in  the 
determination  of  the  positions  occupied  by  those  nuclei.  The  changes  in  position  of  the  nuclei 
in  the  vertebrate  series  appear  to  run  parallel  to  the  changes  in  these  tracts.  The  reader 
will  now  appreciate  the  significance  of  the  close  relation  of  these  nuclei  to  the  medial  longi- 
tudinal and  tectospinal  fasciculi  which  convey  to  them  impulses  from  the  vestibular  and 
optic  centers. 

The  position  of  the  nucleus  of  the  facial  nerve  and  the  curved  course  of  its  fibers  within 
the  pons  may  be  explained  in  the  same  way.  In  a  10  mm.  human  embryo  the  nucleus  ot  the 
facial  nerve  lies  rostral  to  that  of  the  abducens  and  the  motor  fibers  pass  directly  lateralward 


ISO  THE    NERVOUS    SYSTEM 

to  their  exit  from  the  brain  (Fig.  128).  This  nucleus,  which  supplies  the  muscles  that  sur- 
round the  mouth,  receives  axons  from  the  primary  taste  center  in  the  medulla  oblongata 
(the  nucleus  of  the  tractus  solitarius)  which  is  located  at  a  more  caudal  level.  Accordingly, 
the  facial  nucleus  migrates  caudally  toward  that  center.  It  also  receives  fibers  from  the 
nucleus  of  the  spinal  tract  of  the  trigeminal  nerve  and  migrates  ventrolaterally  toward  it. 
Thus  is  explained  the  adult  position  of  the  nucleus  of  the  facial  nerve,  not  far  from  the 
spinal  tract  of  the  trigeminal  nerve  and  near  the  rostral  end  of  the  nucleus  of  the  tractus 
solitarius.  In  the  same  way  the  curved  course  of  the  facial  nerve  within  the  pons  may  be 
explained.  These  examples  are  perhaps  sufficient  to  illustrate  the  general  principle  of  neuro- 
biotaxis. 

Nuclei  of  Origin  and  Terminal  Nuclei. — The  efferent  nuclei,  which  we  have 
examined,  all  have  this  in  common,  that  the  axons,  which  take  origin  from  their 
constituent  cells,  leave  the  brain  through  the  efferent  roots  of  the  cranial  nerves. 
Hence  they  may  all  be  included  under  the  term  nuclei  of  origin.  On  the  other 
hand,  the  afferent  fibers  of  the  cerebrospinal  nerves  have  their  cells  of  origin  located 


© 


Sulcus 


Sulcus 


Genu  internum  n.  facialis 


Sulcus 


Fig.  128. — Diagram  illustrating  three  stages  in  the  development  of  the  genu  of  the  facial 
nerve,  the  youngest,  A,  corresponding  to  the  10  mm.  embryo,  and  the  oldest,  C,  the  newborn 
child.  The  relative  position  of  the  nucleus  of  the  n.  abducens  is  represented  in  outline.  Sulcus, 
Sulcus  medianus  fossae  rhomboideae.     (Streeter,  in  Keibel  and  Mall's  Embryology.) 

outside  the  central  nervous  system  and,  with  the  exception  of  the  first  two  cranial 
nerves,  in  the  cerebrospinal  ganglia.  These  fibers  enter  the  central  nervous 
system  and  end  by  entering  into  synaptic  relations  with  sensory  neurons  of  the 
second  order  located  in  terminal  nuclei.  These  are  classified  according  to  the 
function  of  the  fibers  which  end  in  them  as  visceral  afferent  and  somatic  afferent 
nuclei. 

THE    VISCERAL    AFFERENT    COLUMN 

All  of  the  visceral  afferent  fibers  of  the  cranial  nerves,  except  those  of  the  first 
pair,  are  contained  in  the  facial,  glossopharyngeal,  and  vagus  nerves.  These 
include:  (1)  the  fibers  from  the  taste  buds,  which  since  they  mediate  the  special 
sense  of  taste,  may  be  called  special  visceral  afferent  fibers;  as  well  as  (2)  others 
from  the  posterior  part  of  the  tongue,  and  from  the  pharynx,  larynx,  trachea, 
esophagus,  and  thoracic  and  abdominal  viscera,  which  are  known  as  general 


THE    CRAN1  \l.    NERVES    WD    mill:    \i  CL1  I 


l8l 


visceral  afferent  fibers.  The  majority  of  the  taste  fibers  run  through  the  seventh 
(via  the  chorda  tympani  and  lingual)  and  ninth  nerves  (Cushing,  1903),  but  a 
few  reach  the  epiglottis  by  way  of  the  tenth  (Wilson,  1905     Fig.   129).    All 

of  these  general  and  special  visceral  afferent  libers,  whether  contained  in  the 
seventh,  ninth,  or  tenth  nerves,  enter  the  tractus  solitarius,  within  which  they 
descend  for  varying  distances  (Fig.  120.  yellow).  They  terminate  in  a  column 
of  nerve-cells,  which  in  part  surround  the  tract  and  in  part  are  scattered  among 
its  fibers.  This  is  known  as  the  nucleus  of  the  tractus  solitarius  (Figs.  121,  1  -Id  . 
It  is  a  long  slender  nucleus,  which  extends  throughout  the  entire  length  of  the 
medulla  oblongata  and  is  best  developed  at  the  level  of  origin  of  the  vagus  nerve, 


t      ^W^"}'  ana%t  \\-r  ^3^— ~N ■  max. 

cr&n,ffi 

Gotic. 


Fig.  129. — Diagram  of  the  trigeminal,  facial,  and  glossopharyngeal  nerves  showing  the  course 
of  the  taste  fibers  in  solid  black  lines.  The  broken  and  dotted  lines  indicate  the  course  which  ac- 
cording to  certain  investigators  some  of  the  taste  fibers  are  supposed  to  take:  G.  G.,  Gasserian 
ganglion;  G.  g.,  geniculate  ganglion;  G.  sp.,  sphenopalatine  ganglion;  g.s.p.,  great  superficial  petro- 
sal nerve;  N.  Jac,  the  tympanic  nerve  of  Jacobson;  N,  vid.,  vidian  nerve;  s.s.p.,  small  superficial 
petrosal  nerve.     (Cushing.) 

where  it  lies  ventrolateral  to  the  dorsal  motor  nucleus  of  that  nerve  and  some 
little  distance  below  the  floor  of  the  fourth  ventricle  (Fig.  103).  The  fibers 
from  the  seventh  and  ninth  nerves  terminate  in  the  rostral-  portion  of  the 
nucleus,  which  is  therefore  the  part  especially  concerned  with  the  sense  of  taste, 
while  those  from  the  vagus  end  in  the  caudal  part.  Some  of  these  vagus  fibers 
after  undergoing  a  partial  decussation  terminate  in  a  cell  mass,  the  commissural 
nucleus,  which  lies  dorsal  to  the  central  canal  in  the  closed  part  of  the  medulla 
and  unites  the  nucleus  of  the  tractus  solitarius  on  one  side  with  the  correspond- 
ing nucleus  on  the  other  side  (Fig.  121). 

The  secondary  afferent  paths  from  the  nucleus  of  the  tractus  solitarius  are 
not  well  defined.     Since  gustatory  impulses  arouse  sensations  of  taste  and  the 


l82  THE    NERVOUS    SYSTEM 

afferent  impulses  from  the  viscera  may  be  vaguely  represented  in  conscious- 
ness, there  must  be  a  visceral  afferent  path  to  the  thalamus;  but  concerning 
the  character  and  location  of  this  path  we  are  entirely  ignorant.1  The  fibers 
arising  from  the  nucleus  of  the  tractus  solitarius  enter  the  reticular  formation, 
and  it  is  probable  that  a  majority  of  them  are  distributed  to  the  visceral  motor 
nuclei  of  the  medulla  oblongata,  including  the  nucleus  ambiguus  and  the  dorsal 
motor  nucleus  of  the  vagus.  In  this  way  arcs  are  established  for  a  large  and 
important  group  of  visceral  reflexes.  Some  of  these  fibers  descend  to  the  spinal 
cord  and  may  play  an  important  part  in  the  reflex  control  of  respiration  and 
in  initiating  reflex  coughing  and  vomiting  (Figs.  245,  246). 

THE  GENERAL  SOMATIC  AFFERENT  NUCLEI 
The  general  somatic  afferent  nuclei  receive  fibers  from  the  skin  and  ecto- 
dermal mucous  membrane  of  the  head  by  way  of  the  trigeminal  nerve.  These 
have  their  cells  of  origin  in  the  semilunar  ganglion,  and  within  the  pons  they 
divide  into  short  ascending  and  long  descending  branches  (Fig.  131).  The  as- 
cending branches  terminate  in  the  main  sensory  nucleus;  the  descending  branches 
run  through  the  spinal  tract  and  terminate  in  the  nucleus  of  the  spinal  tract  of 
the  trigeminal  nerve.  Since  these  nuclei  receive  sensory  fibers  from  the  skin 
and  ectodermal  mucous  membrane  of  the  head,  they  are  exteroceptive  in  function. 
The  spinal  tract  and  its  nucleus  also  receives  a  few  cutaneous  afferent  fibers 
through  the  glossopharyngeal  and  vagus  nerves  from  the  skin  of  the  external  ear 
(Fig.  120). 

The  main  sensory  nucleus  of  the  trigeminal  nerve  is  located  at  the  level  of 
the  middle  of  the  pons  in  the  lateral  part  of  the  reticular  formation  some  dis- 
tance from  the  floor  of  the  fourth  ventricle  (Figs.  110,  121,  130).  The  spinal 
nucleus,  with  which  it  is  continuous,  at  first  lies  deeply  under  cover  of  the  resti- 
form  body;  but  when  it  is  traced  caudally  it  approaches  the  surface  and,  covered 
by  the  spinal  tract,  forms  the  tuberculum  cinereum  (Figs.  99,  103).  It  finally 
becomes  continuous  with  the  substantia  gelatinosa  Rolandi  of  the  spinal  cord. 
Thus  we  have  a  continuous  column  of  gray  matter  extending  from  the  sacral  por- 
tion of  the  spinal  cord  into  the  brain  stem  and  ending  abruptly  in  an  enlarge- 
ment, the  main  sensory  nucleus  of  the  trigeminal  nerve.  This  entire  column 
receives  afferent  fibers  from  the  skin  and  belongs  to  the  exteroceptive  portion  of 
the  somatic  ajferent  division  of  the  nervous  system. 

1  Kohnstamm  and  Hindelang  (1910)  and  von  Monakow  (1°13)  have  described  a  secondary 
visceral  afferent  path  which  arises  from  the  gray  matter  in  and  around  the  tractus  solitarius  and 
terminates  in  the  thalamus. 


THE    CRANIAL    NERVES    AND    Till  Ik    \i  I  u  i 


l83 


Secondary  Afferent  Paths.— From  the  cells  of  the  mail]  sensor)  and  spinal 
nuclei  of  the  trigeminal  nerve  arise  fibers  which  enter  the  reticular  formation 
and  are  there  grouped  into  Longitudinal  bundles  from  which  collaterals  are  given 

off  to  the  motor  nuclei  of  the  brain  stem  (  Fig.  131  ).     There  are  at  least  two  su<  li 
longitudinal  bundles  in  each  lateral  half  of  the  brain.     The  ventral  secondary 
afferent  path  of  the  trigeminal  nerve  consists  for  the  most  part  of  crossed  liber- 
and  is  located  in  the  ventral  part  of  the  reticular  formation,  close  to  the  spino 
thalamic  tract  in  the  medulla,  and  dorsal  to  the  medial  lemniscus  in  the  pons 


Mesencephalon 


Pons- 


Ventral  cochlear  nucleus 


Medulla  oblongata 


-**j Cerebral  aqueduct 

-  -  Inferior  eollirulus 

"Mesencephalic  nucleus  of  N.  V 

Sensory  nucleus  of  N.  V 
-/^"Fourth  ventricle 

■-■Vestibular  nucleus 

Dorsal  cochlear  nucleus 

Nucleus  of  tractus  solitarius 

Nucleus  of  spinal  trad  X.  V 


Central  canal 

Fig.  130. — Sensory  nuclei  projected  upon  a  median  sagittal  section  of  the  human  brain  stem. 
Horizontal  lines,  general  somatic  sensory  nuclei;  cross-hatching,  visceral  sensory  nucleus;  stipple, 
special  somatic  sensory  nuclei. 

and  mesencephalon  (Fig.  132).  It  is  composed  in  large  part  of  long  fibers  which 
reach  the  thalamus.  The  dorsal  secondary  afferent  path  of  the  trigeminal  nerve 
consists  chiefly  of  uncrossed  fibers  and  lies  not  far  from  the  floor  of  the  fourth 
ventricle  and  the  central  gray  matter  of  the  cerebral  aqueduct.  It  consist-  in 
considerable  part  of  short  fibers  (Cajal,  1911;  Wallenberg,  1905;  Economo, 
1911;  Dejerine,  1914). 

The  proprioceptive  nuclei  of  the  cranial  nerves  are  not  well  known.  They 
have  to  do  with  afferent  impulses  arising  in  the  muscles  of  mastication  and  in 


1 84 


THE    NERVOUS    SYSTEM 


the  extrinsic  muscles  of  the  eye.     Johnston  (1909)  has  shown  that  the  large 
unipolar  cells  of  the  mesencephalic  nucleus  of  the  fifth  nerve  which  give  rise 


Fig.  131. — Diagram  of  the  nuclei  and  central  connections  of  the  trigeminal  nerve:  A,  Semi- 
lunar ganglion;  B,  mesencephalic  nucleus,  N.  Y.;  C,  motor  nucleus,  N.  Y. ;  D,  motor  nucleus,  N. 
VII;  E,  motor  nucleus,  N.  XII;  F,  nucleus  of  the  spinal  tract  of  N.  V.;  G,  sensory  fibers  of  the  sec- 
ond order  of  the  trigeminal  path:  a,  ascending  and  b,  descending  branches  of  the  sensory  fibers, 
N.  V.;  c,  ophthalmic  nerve;  d,  maxillary  nerve;  e,  mandibular  nerve.     (Cajal.) 

to  the  fibers  of  the  mesencephalic  root  of  that  nerve,  are  probably  sensory  in 
function.     Willems  (1911)  and  Allen  (1919)  believe  that  these  are  sensory  fibers 


THE   CRANIAL   NERVES   AND    iiii.ik    NUCLEI  185 

to  the  muscles  of  mastication.  If  this  interpretation  is  correct  we  are  pre- 
sented with  an  exception  to  the  rule  that  the  afferent  fibers  of  the  cerebrospinal 
nerves  take  origin  from  cells  located  outside  the  cerebrospinal  axis.    This  nu<  leus 

lies  in  the  lateral  wall  of  the  rostral  portion  of  the  fourth  ventricle  and  in  the 
lateral  part  of  the  gray  matter  surrounding  the  cerebral  aqueduct  (Figs.  114, 
121,   130).     The  origin  and  termination  of  the  afferent  fibers  for  the  extrin>ic 


Fig.  132.— Diagram  to  show  the  location  of  the  secondary  sensory  tracts  of  the  trigeminal 
nerve  (solid  black)  in  the  tegmental  portion  of  the  rostral  part  of  the  pons:  B.C.,  Brachium  con- 
junct ivum;  D. T.T.N. ,  dorsal  secondary  sensory  tract  of  the  trigeminal  nerve;  L.L.,  lateral  lemnis- 
cus; M.  L.,  medial  lemniscus;  M.L.F.,  medial  longitudinal  fasciculus;  V.T.T.N.,  ventral  secondary 
sensory  tract  of  trigeminal  nerve. 

muscles  of  the  eye  are  unknown,  although  we  know  that  such  afferent  fibers 
are  present  in  the  oculomotor,  trochlear,  and  abducens  nerves. 

SPECIAL  SOMATIC  AFFERENT  NUCLEI 

The  special  somatic  afferent  nuclei  are  associated  with  the  acoustic  nerve, 
which  is  composed  of  two  divisions.  One  part,  the  cochlear  nerve,  conveys  im- 
pulses aroused  by  sound  waves  reaching  the  cochlea  through  the  outer  ear 
and  tympanic  cavity.  Since  it  responds  to  stimuli  from  without,  the  cochlear 
apparatus  subserves  exteroceptive  functions.  The  vestibular  nerve,  on  the  other 
hand,  conveys  impulses  from  the  semicircular  canals  of  the  ear.  These  are  im- 
portant proprioceptive  sense  organs  and  give  information  concerning  the  move- 
ments and  posture  of  the  head. 

The  cochlear  nuclei  are  the  terminal  nuclei  of  the  cochlear  nerve,  the  fibers 
of  which  take  origin  in  the  spiral  ganglion  of  the  cochlea.  This  is  composed  of 
bipolar  cells,  each  having  a  short  peripheral  and  a  longer  central  process  (Fig. 
133).  The  peripheral  process  terminates  in  the  spiral  organ  of  Corti.  The 
central  process  is  directed  toward  the  brain  in  the  cochlear  nerve.  These  central 
fibers  terminate  in  two  masses  of  gray  matter,  located  on  the  restiform  body  near 
the  point  where  the  latter  turns  dorsally  into  the  cerebellum  (Figs.  107,  121, 


i86 


THE    NERVOUS    SYSTEM 


130).  One  of  these  masses,  the  dorsal  cochlear  nucleus,  is  placed  on  the  dorso- 
lateral aspect  of  the  restiform  body  and  produces  a  prominent  elevation  on  the 
surface  of  the  brain  (Fig.  91).  The  other,  known  as  the  ventral  cochlear  nucleus, 
is  in  contact  with  the  ventrolateral  aspect  of  the  restiform  body. 

Secondary  Auditory  Path. — From  the  cells  of  the  ventral  cochlear  nucleus 
arise  fibers  which  stream  medialward  in  the  ventral  part  of  the  pars  dorsalis 
pontis  and  form  the  trapezoid  body  (Figs.  108,  134).  The  fibers  cross  the  median 
plane  and  on  reaching  the  lateral  border  of  the  opposite  superior  olivary  nucleus 
turn  rostrally  as  a  compact  bundle  known  as  the  lateral  lemniscus  (Figs.  110, 


Fig.  133. — Section  of  the  spiral  ganglion  and  organ  of  Corti  of  the  mouse:  A,  Bipolar  cells  of 
the  spiral  ganglion;  B,  outer  hair  cells;  C,  sustentacular  cells;  D,  terminal  arborization  of  the 
peripheral  branch  of  a  bipolar  cell  about  an  inner  hair  cell;  T,  tectorial  membrane.  Golgi  method. 
(Cajal.) 

112,  114).  Some  of  the  fibers  of  the  trapezoid  body  end  in  the  superior  olivary 
nuclei  and  in  the  nuclei  of  the  trapezoid  body,  while  others  give  off  collaterals  to 
these  nuclear  masses.  Some  of  the  fibers  arising  in  these  nuclei,  especially  in 
the  nuclei  of  the  trapezoid  body,  join  in  the  formation  of  the  lateral  lemniscus; 
but  according  to  Cajal  (1909)  a  majority  of  the  fibers  from  the  superior  olivary 
nucleus  belong  to  short  reflex  pathways  in  the  reticular  formation  connecting 
the  cochlear  nerve  with  the  nuclei  of  the  motor  nerves  of  the  head  and  neck. 
Fibers  arising  in  the  dorsal  cochlear  nucleus,  and  possibly  also  some  from  the 
ventral  cochlear  nucleus,  sweep  over  the  dorsal  surface  of  the  restiform  body 
and  the  floor  of  the  fourth  ventricle  as  the  strice  medullares  acusticce.     These  may 


THi:    CRANIAL    XKRVKS    AM)    TIIKIK     \l  (   l.l.l 


l87 


lie  just  beneath  the  ependyma  or  may  be  buried  in  the  gray  matter  of  the  rhom- 
boid fossa.  On  reaching  the  median  plane  the>e  fibers  decussate,  ^ink  into  the 
reticular  formation,  and  join  the  trapezoid  body  or  lateral  lemniscus  of  the 
opposite  side.  Some  probably  fail  to  cross,  since  clinical  experience  and  evi- 
dence based  on  animal  experiments  tend  to  show  that  a  part  of  the  fibers  in  the 
lateral  lemniscus  represent  an  uncrossed  path  from  the  cochlear  nuclei  of  the 
same  side  (Kreidl,  1914). 

Transverse  temporal  gyms 

Auditory  radiation 

Medial  geniculate  body 
Inferior  colliculus 


Lateral  lemnisci 


Collaterals  to  nucleus  of 
lateral  lemniscus 


Rostral  portion  of  the  pons 


/Stria:  medullares 


,  Dorsal  cochlear  nucleus 

"Ventral  coclilear  nucleus 
Cochlear  nerve 
>  Vestibular  nerve 


Caudal  portion  of  the  pons--\- 

Superior  olive  ■ 

Trapezoid  body ' 

Nucleus  of  the  trapezoid  body 
Fig.  134. — Diagram  of  the  auditory  pathway.     (Based  on  the  researches  of  Cajal  and  Kreidl.) 

As  the  lateral  lemniscus  ascends  in  the  reticular  formation  of  the  pons,  there 
are  scattered  among  its  fibers  many  nerve-cells  which  together  constitute  the 
nucleus  of  the  lateral  lemniscus.  To  these  cells  it  gives  off  collaterals  and  pos- 
sibly also  terminal  branches,  and  from  them  it  is  said  to  receive  additional  fibers. 
But  according  to  Cajal  the  axons  arising  here  do  not  ascend  in  the  lateral  lem- 
niscus, but  are  directed  medially  into  the  reticular  formation. 

On  reaching  the  mesencephalon  the  lateral  lemniscus  terminates  in  part  in 
the  inferior  colliculus,  but  also  sends  branches  and  direct  fibers  by  way  of  the 
inferior  quadrigeminal  brachium  to  the  medial  geniculate  body.     While  the  me- 


1 88  THE    NERVOUS    SYSTEM 

dial  geniculate  body  is  a  way-station  on  the  auditory  path  to  the  cerebral  cor- 
tex, the  inferior  colliculus  serves  as  a  center  for  reflexes  in  response  to  sound. 
The  Vestibular  Nuclei. — The  fibers  of  the  vestibular  nerve  take  origin  from 
the  bipolar  cells  of  the  vestibular  ganglion  located  in  the  internal  auditory  meatus 
(Fig.  135).  The  cochlear  and  vestibular  divisions  of  the  acoustic  nerve  sepa- 
rate at  the  ventral  border  of  the  restiform  body.     Here  the  vestibular  nerve 


Fig.  135. — The  vestibular  ganglion  and  the  termination  of  the  peripheral  branches  of  its  bi- 
polar cells  in  a  macula  acustica:  A,  Hair  cells  and  B,  sustentacular  cells  of  the  macula;  D,  terminal 
arborization  of  the  peripheral  branches  of  the  bipolar  cells  of  the  vestibular  ganglion  (G)  about  the 
hair  cells  of  the  macula;  F,  facial  nerve;  R,  central  branches  of  the  bipolar  cells  directed  toward  the 
medulla  oblongata  T.     Mouse.     Golgi  method.     (Cajal.) 

penetrates  into  the  brain,  passing  between  the  restiform  body  and  the  spinal 
tract  of  the  trigeminal  nerve  toward  the  area  acustica  of  the  rhomboid  fossa. 
Under  cover  of  the  area  acustica  the  fibers  divide  into  short  ascending  and 
longer  descending  branches  (Figs.  134,  136).  There  may  be  enumerated  five 
cellular  masses  within  which  these  fibers  terminate,  namely:  (1)  the  principal 
or  medial  nucleus,  (2)  the  descending  or  spinal  nucleus,  (3)  the  superior  nucleus 


THE    CRANIAL    NERVES    AND    IIIKIK    NUCLEI 


of  Bechterew,  (4)  the  lateral  amicus  of  Deiters,  and  (5)  the  cerebellum    I 
130,  136). 

The  principal,  medial,  or  dorsal  vestibular  nucleus  is  very  large.  1 1  lies  sub- 
jacent to  the  major  portion  of  the  area  acustica  and  belongs,  therefore,  to  both 
the  pons  and  the  medulla  oblongata  (Figs.  89,  103,  107).  The  -ray  matter. 
associated  with  the  descending  branches  from  the  vestibular  nerve  and  lying  on 
the  medial  side  of  the  restiform  body,  constitutes  the  spinal  or  descending 
vestibular  nucleus.     Along  with  the  descending  fibers  it  can  be  followed  in  serial 


Nuc.  of  oculomotor 
nerve 


Nuc.  of  trochlear  nerve  —  -^~- 


Brachium  pontis^^ 


Xuc.  of  abducens  nerve. 
Rhomboid  fossa- 
Medulla  oblongata. 


Superior  colliculus 


Inferior  colliculus 


Med.  longitudinal 
fast  ii  ulus 

Superior  vestibular 


I  'eslibuloccrcbcllar 
tract 


f— Lateral  vestibular  nuc. 


Vestibular  nerve 


'  Spinal  vestibular  nuc. 

•Principal  vestibular 
nucleus 


Fig.  136. — Diagram  of  the  nuclei  and  central  connections  of  the  vestibular  nerve.     (Based  on 

figures  by  Herrick  and  Weed.) 

sections  as  far  as  the  rostral  extremity  of  the  nucleus  gracilis.  The  lateral  vestib- 
ular nucleus  of  Deiters  is  situated  close  to  the  restiform  body  at  the  point  where 
the  fibers  of  the  vestibular  nerve  begin  to  diverge  (Fig.  107).  It  is  composed  of 
large  multipolar  cells  like  those  found  in  motor  nuclei.  Directly  continuous  with 
the  medial  and  lateral  nuclei  is  a  mass  of  medium-sized  cells,  the  superior  vestib- 
ular nucleus  of  Bechterew,  located  in  the  floor  and  lateral  wall  of  the  fourth 
ventricle  lateral  to  the  abducens  nucleus,  and  the  emergent  fibers  of  the  facial 
nerve  (Fig.  108).  It  extends  as  far  rostrad  as  the  caudal  border  of  the  main 
sensory  nucleus  of  the  trigeminal  nerve  (Weed,  1914). 


190  THE    NERVOUS    SYSTEM 

Many  of  the  ascending  branches  of  the  vestibular  nerve,  after  giving  off 
collaterals  to  the  nuclei  of  Deiters  and  Bechterew,  are  prolonged  in  the  tractus 
vestibulocerebellaris,  to  end  in  the  cortex  of  the  cerebellum  (Cajal,  1909).  These 
are  joined  by  fibers  arising  in  the  superior  and  lateral  vestibular  nuclei  which 
also  run  to  the  cerebellum  (Fig.  136).  From  the  standpoint  of  its  embryologic 
development  the  cerebellum  may  properly  be  regarded  as  a  highly  specialized 
vestibular  nucleus  (p.  196). 

Secondary  Vestibular  Paths. — In  addition  to  the  fibers  to  the  cerebellum 
mentioned  in  the  preceding  paragraph  two  important  tracts  of  fibers  take  origin 
in  the  superior  and  lateral  vestibular  nuclei.  One  of  these  was  encountered  in 
the  study  of  the  medial  longitudinal  bundle.  Cells  in  the  superior  and  lateral 
vestibular  nuclei  give  rise  to  fibers  which  run  to  the  medial  longitudinal  fascicle 
of  the  same  and  of  the  opposite  side,  and  through  it  reach  the  motor  nuclei  of  the 
ocular  muscles  (Fig.  136).  In  this  way  there  is  established  an  arc,  which  makes 
possible  the  reflex  response  of  the  eye  muscles  to  afferent  impulses  arising  in  the 
vestibule  and  semicircular  canals  of  the  ear.  The  other  bundle  was  considered 
in  connection  with  the  spinal  cord  as  the  vestibulospinal  tract,  the  fibers  of 
which  take  origin  from  the  cells  of  the  lateral  nucleus  and  descend  into  the 
anterior  funiculus  of  the  same  side  of  the  cord.  These  fibers  serve  to  place  the 
primary  motor  neurons  of  the  spinal  cord  under  the  reflex  control  of  the  vestib- 
ular apparatus. 

From  the  medial  border  of  the  principal  vestibular  nucleus  many  scattered 
fibers  cross  the  raphe  and  enter  the  reticular  formation  of  the  opposite  side,  where 
they  become  longitudinal  fibers.  No  tract  to  the  thalamus  is  known,  a  fact 
which  is  in  keeping  with  this  other,  that  ordinarily  the  activities  of  the  vestib- 
ular apparatus  are  not  clearly  represented  in  consciousness. 

SUMMARY  OF  THE  ORIGIN,  COMPOSITION,  AND  CONNECTIONS  OF  THE  CRANIAL 

NERVES 

The  olfactory  and  optic  nerves  and  the  nervus  terminalis,  which  have  not 
yet  been  considered  in  detail,  have  been  included  in  this  summary  for  the  sake 
of  completeness. 

The  nervus  terminalis  is  a  recently  discovered  nerve  which  arises  from  the 
cerebral  hemisphere  in  the  region  of  the  medial  olfactory  tract  or  stria.  It  is 
closely  associated  with  the  olfactory  nerve  and  its  fibers  run  to  the  nasal  septum. 
The  origin,  termination,  and  function  of  its  component  fibers  are  not  yet  under- 
stood (McKibben,  1911;  Huber   and    Guild,  1913;  McCotter,  1913;  Johnston, 


THE    CRANIAL    NER\  ES     \\n     Mil  1 R    \i  .  1.1  [ 


191 


1914;  Brookover,  1914,  1(>17;  Larsell,  1918,  1919).    Since  it  was  unknown  at 
the  time  the  cranial  uerves  were  first  enumerated,  it  bears  no  numerical  d 

nation. 

I.  Olfactory  Nerve.  Superficial  origin  from  the  olfactory  hull)  in  the  form  of 
a  number  of  line  fila  which  separately  pass  through  the  openings  in  the  cribri- 
form plate.  It  is  composed  of  special  visceral  afferent  fibers  with  cells  of  origin 
in  tin-  olfactory  mucous  membrane.  The  fibers  terminate  in  the  glomeruli  of 
the  olfactory  bulb. 

II.  Optic  Nerve.-  Not  a  true  nerve;  but  both  from  the  standpoint  of  its 
structure  and  development  a  fiber  tract  of  the  brain.  Superficial  origin,  from  the 
optic  chiasma.  or  after  partial  decussation,  from  the  lateral  geniculate  body, 
pulvinar  of  the  thalamus,  and  superior  colliculus.  Component  fibers^,  special 
somatic  afferent — exteroceptive;  origin,  ganglion  cells  of  the  retina;  terminations 
in  the  lateral  geniculate  body,  pulvinar  of  the  thalamus  and  superior  colliculus. 
The  fibers  from  the  nasal  half  of  each  retina  cross  in  the  optic  chiasma.1 

III.  Oculomotor  Nerve. — Superficial  origin,  from  the  oculomotor  sulcus  on 
the  medial  aspect  of  the  cerebral  peduncle.     Composition: 

1.  Somatic  Efferent  Fibers. — Cells  of  origin,  in  the  oculomotor  nucleus  of  the 
same  and  to  a  less  extent  of  the  opposite  side  (Fig.  120).  Termination,  in  the 
extrinsic  muscles  of  the  eye  except  the  superior  oblique  and  the  lateral  rectus. 

2.  General  Visceral  Efferent  Fibers. — Cells  of  origin  in  the  Edinger-Westphal 
nucleus.  Termination  in  the  ciliary  ganglion,  from  the  cells  of  which  post- 
ganglionic fibers  run  to  the  intrinsic  muscles  of  the  eye.2 

IV.  Trochlear  Nerve. — Superficial  origin,  from  the  anterior  medullary  ve- 
lum. Composed  of  somatic  efferent  fibers;  cells  of  origin  in  the  trochlear  nucleus; 
decussation  in  the  anterior  medullary  velum;  termination  in  the  superior  oblique 
muscle  of  the  eye  (Fig.  120). 

V.  Trigeminal  Nerve  .—Superficial  origin,  from  the  lateral  aspect  of  the 
middle  of  the  pons  by  two  roots:  the  portio  major  or  sensory  root  and  the  portio 
minor  or  motor  root.     Composition  (Fig.  120): 

1.  General  Somatic  Afferent  Fibers. — A,  Exteroceptive — Cells  of  origin  in  the 
semilunar  ganglion  (Gasserii),  chiefly  unipolar  with  T-shaped  axons,  peripheral 

1  It  has  been  demonstrated  by  Arey  that  there  are  also  efferent  fibers  in  the  optic  nerves  of 
fishes  which  control  the  movement  of  the  retinal  elements  in  response  to  light,  Jour.  Com  p.  Xeur., 
vol.  26,  p.  213. 

2  It  is  probable  that  the  oculomotor,  trochlear,  and  abducens  nerves  contain  proprioceptive 
fibers  for  the  extrinsic  muscles  of  the  eye,  but  the  cells  of  origin  and  the  central  connections  of 
these  sensory  components  are  unknown. 


192  THE    NERVOUS    SYSTEM 

branches  to  skin  and  mucous  membrane  of  the  head,  central  branches  by  way 
of  the  portio  major  to  the  brain.  Termination  in  the  main  sensory  nucleus  and 
nucleus  of  the  spinal  tract  of  the  trigeminal  nerve. 

2.  General  Somatic  Afferent  Fibers. — B,  Proprioceptive — Cells  of  origin  prob- 
ably located  in  the  mesencephalic  nucleus  of  the  fifth  nerve.  Fibers  by  way 
of  the  portio  major,  distributed  as  sensory  fibers  to  the  muscles  of  mastication. 

3.  Special  Visceral  Efferent  Fibers. — Cells  of  origin  in  the  motor  nucleus  of 
the  fifth  nerve.  Fibers  by  way  of  the  portio  minor  and  the  mandibular  nerve 
to  the  muscles  of  mastication. 

VI.  Abducens  Nerve. — Superficial  origin,  from  the  lower  border  of  the 
pons  just  rostral  to  the  pyramid.  Composed  of  somatic  efferent  fibers;  cells  of 
origin  in  the  abducens  nucleus;  termination  in  the  lateral  rectus  muscle  of  the 
eye. 

VII.  Facial  Nerve  and  Nervus  Intermedius. — Superficial  origin  from  the 
lateral  part  of  the  lower  border  of  the  pons  separated  from  the  flocculus  by  thex 
eighth  nerve.     Composition  (Fig.  120) : 

1.  Special  Visceral  Afferent  Fibers. — Cells  of  origin  in  the  ganglion  geniculi, 
chiefly  unipolar,  with  T-shaped  axons.  The  peripheral  branches  run  by  way  of 
the  chorda  tympani  and  lingual  nerves  to  the  taste  buds  of  the  anterior  two- 
thirds  of  the  tongue.  The  central  branches  run  by  way  of  the  nervus  intermedius 
to  the  tractus  solitarius  and  end  in  the  nucleus  of  that  tract.  It  is  probable  that 
the  taste  fibers  terminate  in  the  rostral  part  of  this  nucleus.1 

2.  General  Visceral  Efferent  Fibers. — Cells  of  origin  in  the  nucleus  salivatorius 
superior.  These  fibers  run  by  way  of  the-  nervus  intermedius,  facial  nerve, 
chorda  tympani,  and  lingual  nerve  to  the  submaxillary  ganglion  for  the  in- 
nervation of  the  submaxillary  and  sublingual  salivary  glands. 

3.  Special  Visceral  Efferent  Fibers. — Cells  of  origin  in  the  motor  nucleus  of 
the  facial  nerve.  These  fibers  run  by  way  of  the  facial  nerve  to  end  in  the  super- 
ficial musculature  of  the  face  and  scalp,  and  in  the  platysma,  posterior  belly  of 
the  digastric,  and  stylohyoid  muscles. 

VIII.  Acoustic  Nerve. — Superficial  origin  from  the  lateral  part  of  the  lower 
border  of  the  pons  near  the  flocculus.  Consists  of  two  separate  parts  known  as 
the  vestibular  and  cochlear  nerves. 

1  Herrick  (1918)  describes  general  visceral  afferent  fibers  in  the  facial  nerve  which  he  says 
mediate  deep  visceral  sensibility  and  are  probably  found  in  all  the  branches  of  the  facial.  And 
Rhinehart  (1918)  has  described  a  cutaneous  branch  of  the  facial  in  the  mouse.  This  branch  con- 
tains general  somatic  afferent  fibers,  which  arise  in  the  geniculate  ganglion  and  terminate  in  the 
skin. 


I  111      CB  \M  M.    M  l^\  ES     \\l»     I  III.IK     \l  I   III 


The  Vestibular  Nerve.  The  componenl  Gibers  belong  to  the  special  som<iti< 
rent  group  and  arc  proprioceptive.  Cell-  of  origin,  in  the  vestibular  ganglion, 
arc  bipolar.  Their  peripheral  branches  run  to  the  5emi<  ir<  ular  i  anal  .  utri<  L< 
saccule.  'Their  central  branches  terminate  in  the  principal,  lateral  superior,  and 
spinal  vestibular  nuclei.  Some  of  them  run  withoul  interruption  to  the  <  erebellum. 
The  Cochlear  Nerve.  The  component  fibers  belong  to  tin-  special  somatic 
afferent  group  and  are  exteroceptive.  Cells  of  origin,  in  the  spiral  ganglion  of 
the  cochlea,  are  bipolar.  Their  peripheral  branches  end  in  the  spiral  organ  of 
Cot  ti.    Their  central  branches  terminate  in  the  ventral  and  dorsal  <  o<  hlear  nuclei. 

IX.  The  Glossopharyngeal  Nerve.  Superficial  origin,  from  the  rostral 
end  of  the  posterior  lateral  sulcus  of  the  medulla  oblongata  in  line  with  the 
tenth  and  eleventh  nerves.     Composition  (Fig.  120): 

1.  General  Visceral  Afferent  Fibers. — Cells  of  origin  in  the  ganglion  petrosum, 
peripheral  branches  form  the  general  sensory  fibers  to  the  pharynx  and  posterior 
third  of  the  tongue,  central  branches  run  to  the  tractus  solitarius  and  its  nucleus. 

2.  Special  Visceral  Afferent  Tnfors.— Cells  of  origin  in  the  ganglion  petrosum, 
peripheral  branches  to  the  taste  buds  of  the  posterior  third  of  the  tongue,  central 
branches,  to  the  tractus  solitarius  and  its  nucleus. 

3.  General  Visceral  Efferent  Fibers. — Cells  of  origin  in  the  inferior  salivatory 
nucleus;  fibers  run  to  the  otic  ganglion,  from  the  cells  of  which  postganglionic 
fibers  carry  the  impulses  to  the  parotid  gland. 

4.  Special  Visceral  Efferent  Fibers. — Cells  of  origin  in  the  nucleus  ambiguus. 
Termination  in  the  stylopharyngeus  muscle. 

X.  Vagus  Nerve. — Superficial  origin  from  the  rostral  part  of  the  posterior 
lateral  sulcus  of  the  medulla  oblongata  in  line  with  the  ninth  and  eleventh  and 
just  caudal  to  the  ninth.     Composition  (Fig.  120): 

1.  General  Somatic  Afferent  Fibers. — Cells  of  origin  in  the  ganglion  jugulare; 
peripheral  branches  to  the  skin  of  the  external  ear  by  way  of  the  ramus  auricularis; 
central  branches  to  the  spinal  tract  of  the  trigeminal  nerve  and  its  nucleus. 
According  to  Herrick,  some  of  these  fibers  from  the  external  ear  run  by  way  of 
the  glossopharyngeal  nerve  also. 

2.  General  Visceral  Afferent  Fibers.— Cells  of  origin  in  the  ganglion  nodosum; 
peripheral  branches  run  as  sensory  fibers  to  the  pharynx,  larynx,  trachea,  esopha- 
gus, and  the  thoracic  and  abdominal  viscera;  central  branches  run  to  the  tractus 
solitarius  and  terminate  in  its  nucleus.1 

1  According  to  Wilson  (1905)  there  are  also  special  visceral  afferent  fibers  in  the  vagus  for 
the  taste  buds  of  the  epiglottis.     These  also  terminate  in  the  nucleus  of  the  tractus  solitarius. 

13 


194 


THE    NERVOUS    SYSTEM 


3.  General  Visceral  Efferent  Fibers. — Cells  of  origin  in  the  dorsal  motor  nucleus 
of  the  vagus.  Fibers  run  to  the  sympathetic  ganglia  of  the  vagal  plexuses  for 
the  innervation  of  the  thoracic  and  abdominal  viscera. 

4.  Special  Visceral  Efferent  Fibers. — Cells  of  origin  in  the  nucleus  ambiguus. 
Termination  in  the  striated  musculature  of  the  pharynx  and  larynx. 

XL  Accessory  Nerve. — Superficial  origin  from  the  posterior  lateral  sulcus 
of  the  medulla  oblongata  caudal  to  the  ninth  and  tenth  and  from  the  lateral  as- 
pect of  the  first  five  or  six  cervical  segments  of  the  spinal  cord.  Composition 
(Fig.  120): 

1.  General  Visceral  Efferent  Fibers. — Cells  of  origin  in  the  dorsal  motor 
nucleus  of  the  vagus.  Fibers  run  in  the  bulbar  rootlets  and  then  by  way  of  the 
internal  ramus  of  the  accessory  to  join  the  vagus,  and  end  in  the  sympathetic 
plexuses,  associated  with  the  vagus  nerve,  for  the  innervation  of  thoracic  and 
abdominal  viscera. 

2.  Special  Visceral  Efferent  Fibers. — These  fall  into  two  groups:  A,  fibers, 
whose  cells  of  origin  are  located  in  the  nucleus  ambiguus,  and  which  run  by  way  of 
the  internal  ramus  of  the  accessory  to  join  the  vagus  and  are  distributed  through 
it  to  the  striated  muscles  of  the  pharynx  and  larynx;  B,  fibers,  whose  cells  of 
origin  lie  in  the  lateral  part  of  the  anterior  gray  column  of  the  first  five  or  six 
cervical  segments  of  the  spinal  cord,  and  which  ascend  in  the  spinal  root  of  the 
accessory  nerve  and  then  run  in  its  external  ramus  to  end  in  the  trapezius  and 
the  sternocleidomastoid  muscles. 

XII.  Hypoglossal  Nerve. — Superficial  origin  from  the  anterior  lateral  sulcus 
of  the  medulla  between  the  pyramid  and  the  olive.  It  is  composed  of  somatic 
efferent  fibers,  whose  cells  of  origin  are  located  in  the  hypoglossal  nucleus  and 
whose  termination  is  in  the  musculature  of  the  tongue. 


CHAPTER  XIII 


THE  CEREBELLUM 


DEVELOPMENT  OF  THE  CEREBELLUM 


The  dorsal  border  of  the  alar  lamina  occupies  a  lateral  position  in  the  rhom- 
bencephalon and,  as  a  result  of  the  development  of  the  pontine  flexure,  acquires 
a  V-shaped  bend  at  the  apex  of  which  is  the  lateral  recess  of  the  fourth  ventricle 
(Fig.   137,  A).     This  dorsal  border  becomes  everted  and  forms  a  prominent 


Cerebellum  ^ 


Lateral  recess 


Rhombic  lip 


Corpora  quadrigemina 
Cerebrum 


Anlage  of 
vermis 

Lateral  lobe  of 
cerebellum 


Rhombic  lip 


Mid-brain 


Medulla 
oblongata 


Obex 


W^ 


Lateral  lobe  of  cerebellum     Lobules  of  vermis 


Flocculus  \ 


D 

Lateral  lobe  of        Pyramis 
cerebellum 


Flocculus 


Uvula 


Nodulus 


Fig.  137. — Dorsal  view  of  four  stages  in  the  development  of  the  cerebellum:  A,  of  a  13.6 
mm.  embryo  (His);  B,  of  a  24  mm.  embryo;  C,  of  a  110  mm.  fetus;  D,  of  a  150  mm.  fetus.  (Pren- 
tiss and  Arey.) 

ridge  known  as  the  rhombic  lip.  From  the  portion  of  this  ridge  caudal  to  the 
lateral  recess  develop  the  taenia  of  the  fourth  ventricle  and  the  obex.  At  the 
level  of  the  recess  the  fibers  of  the  acoustic  nerve  reach  the  dorsal  edge  of  the 
alar  lamina,  which,  accordingly,  undergoes  development  at  this  point  into 
vestibular  and  cochlear  nuclei.     More  rostrally  it  undergoes  an  excessive  devel- 

195 


ig6  THE   NERVOUS   SYSTEM 

opment,  which  is  stimulated  by  the  growth  into  it  of  afferent  fibers  from  the 
vestibular  nerve  and  of  sensory  fibers  of  the  second  order,  bringing  afferent 
impulses  from  other  sources,  chiefly  from  the  somatic  musculature.  This 
part  of  the  alar  lamina,  which  may  be  regarded  as  an  overgrown  portion  of  the 
vestibular  nucleus,  develops  into  the  cerebellum.  As  the  paired  cerebellar  plates 
increase  in  thickness  during  the  second  month  of  embryonic  development,  they 
bulge  inward  toward  the  ventricle  and  take  up  a  transverse  position  (Fig.  137, 
B).  As  they  increase  in  size  they  invade  the  roof  plate  and  unite  in  the  median 
plane  forming  a  transverse  bar  above  the  fourth  ventricle.  The  lateral  ex- 
tremities of  this  bar  expand,  and  the  entire  structure  assumes  a  dumb-bell 
shape,  the  lateral  masses  representing  the  future  cerebellar  hemispheres  and  the 
intermediate  part  the  future  vermis. 

At  the  close  of  the  third  month  transverse  sulci  begin  to  appear  in  the  vermis. 
The  first  of  these,  the  fissura  prima  or  sulcus  primarius,  extends  into  the  lateral 
masses  on  either  side  and  separates  an  anterior  lobe  from  the  remainder  of  the 
cerebellum.  Other  transverse  fissures  soon  appear,  due  to  the  rapid  expansion 
and  resultant  folding  of  the  cortical  layers. 

The  cerebellum  differs  from  the  other  parts  of  the  nervous  system,  which  we 
have  thus  far  studied  in  detail,  in  that  the  relative  position  of  the  gray  and  white 
matter  is  reversed.  The  gray  substance  forms  a  thin  superficial  layer,  the 
cerebellar  cortex,  which  covers  a  central  white  medullary  body  (corpus  medullare). 
Originally  the  cerebellar  plate  is  formed,  like  other  parts  of  the  neural  tube,  of 
an  ependymal,  a  nuclear  or  mantle,  and  a  cell-free  marginal  zone.  The  neuro- 
blasts of  the  mantle  zone  take  no  part  in  the  formation  of  the  cortex,  but  become 
grouped  in  the  internal  nuclear  masses  of  the  cerebellum.  The  superficial  or 
marginal  zone  is  at  first  devoid  of  nuclei;  the  neuroblasts,  from  which  the  cere- 
bellar cortex  is  differentiated,  migrate  into  this  zone  from  the  ependymal  and 
perhaps  also  from  the  mantle  layers  of  the  rhombic  lip.  These  developing  neu- 
rons send  their  axons  inward  instead  of  outward  as  in  the  case  of  the  spinal  cord. 
These  axons  accumulate,  along  with  others  which  enter  the  cerebellum  from 
without,  in  the  deep  part  of  the  marginal  layer  and  form  the  central  medullary 
body  of  the  cerebellum,  separating  the  developing  cortex  from  the  deep  nuclear 
masses  that  are  differentiating  from  the  mantle  layer. 

THE  ANATOMY  OF  THE  CEREBELLUM 

It  is  customary  to  consider  the  cerebellum  as  composed  of  three  parts:  a 
small  unpaired  median  portion,  called  the  vermis,  because  superficially  it  re- 


THE    CEREBELLI  M 


ltH 


sembles  a  worm  bent  on  itself  to  form  almost  a  complete  circle;  and  two  large 
lateral  masses,  the  cerebellar  hemispheres,  which  are  connected  with  cadi  other 
by  the  vermis  ( Figs.  138,  139).  Although  morphologically  incorrect,  this  ub- 
division  has  the  advantage  of  convenience  as  well  as  of  established  usage.  On 
the  rostral  aspect  of  the  cerebellum  the  vermis  forms  a  median  ridge,  nol  sharply 
marked  off  laterally  from  the  hemispheres.  'This  part  has  been  (ailed  the  superior 
vermis,  and  in  contradistinction  the  remainder  is  known  as  the  inferior  vermis. 
The  latter  forms  a  prominent  ridge,  marked  off  from  the  hemisphere  on  either 
side  by  a  well-delined  sulcus.  It  lies  in  a  deep  groove  between  the  hemispheres, 
known  as  the  vallecula,  within  which  the  medulla  oblongata  is  lodged.  The 
hemispheres  are  also  partially  separated  from  each  other  by  deep  notches,  the 

Anterior  cerebellar  notch        Central  lobule 

/      Ala  of  central  lobule 

Quaa'rangn-j         Ant,  portion..  ^^X.^, ^/Sj9jr>>^        Culmen        r  ■     , 

tar  lobule    {PosL  p:irlillll  5SS=|^  fl^v    Declhe>      m°"l'C"l"S 

Cerebellar  hem  i-  w  -  -  Primary  fissure 

sphere  superior  j 

surface  MMR  \  \ 


Superior  semi- 
lunar lobule 


Cerebellar  folia^^^B^ 
Inferior  semilunar  lobule 


'  -Post clival  sulcus 
nzontal  cerebellar  sulcus 


Folium  of  vermis 
Posterior  cerebellar  notch 


Fig.  138. — Dorsal  view  of  the  human  cerebellum.      (Modified  from  Sobotta-McMurrich.) 

incisures  cerebelli.  The  anterior  cerebellar  notch  (semilunar  notch)  is  broad  and 
deep;  and  as  seen  from  above  it  is  occupied  by  the  brachia  conjunctiva  and 
the  inferior  colliculi  of  the  corpora  quadrigemina.  The  posterior  cerebellar 
notch  (marsupial  notch)  is  smaller,  and  within  it  is  lodged  a  fold  of  the  dura 
mater,  the  falx  cerebelli. 

The  superior  vermis  is  divided  by   transverse  fissures  into  the  following 
lobules  (Fig.  138): 

1.  Lingula,  closely  applied  to  the  anterior  medullary  velum  between  the  two 
brachia  conjunctiva. 

2.  Central  lobule,  associated  with  the  small  alae  lobuli  centralis  of  the  hemi- 
sphere. 


198 


THE    NERVOUS    SYSTEM 


3.  Monticulus.  which  is  further  subdivided  into  the  c  id  men  and  declive.  The 
former  goes  over  laterally  without  line  of  demarcation  into  the  anterior  portion 
of  the  quadrangular  lobule,  and  the  latter  into  the  posterior  portion  of  the  same 
lobule  in  the  hemisphere. 

4.  Folium  vermis  at  the  posterior  extremity  of  the  superior  vermis. 

The  rostral  or  dorsal  surface  of  the  hemisphere  is  subdivided  by  curved 
transverse  fissures,  which  are  continued  across  the  vermis,  into  the  following 
parts : 

1.  The  anterior  part  of  the  quadrangular  lobule,  continuous  with  the  oilmen 
monticuli  of  the  vermis. 

2.  The  posterior  part  of  the  quadrangular  lobule,  continuous  with  the  declive 
monticuli. 

Nodule  of  vermis       Flocculus 


Inferior  n  rmis 


Cerebellar  hemisphere 
inferior  surface** 


Tonsil 

-  Bivenlral  lobule 


Inferior  semi- 
lunar lobule 

Horizontal  cere- 
bellar sulcus 

Superior  semilunar 
lobule 


Ui  da  of  vermis  '         /  poster;or    '      N  Tuber  of  vermis 
Pyramid  of  vermis'    cerebellar      Folium  of  vermis 
notch 

Fig.  139.— Ventral  view  of  the  human  cerebellum.     (Sobotta-McMurrich.) 


3.  The  superior  semilunar  lobule,  occupying  a  large  crescentic  area  along  the 
dorsolateral  border  of  the  rostral  surface. 

The  inferior  vermis  (Fig.  139)  is  divided  by  transverse  sulci  into  the  follow- 
ing lobules : 

1.  The  tuber  vermis,  next  to  the  folium. 

2.  The  pyramis. 

3.  The  inula. 

4.  The  nodidus. 

The  caudal  surface  of  the  hemisphere  presents  the  following  subdivisions : 
1.  The  inferior  semilunar  lobule,  occupying  a  large  part  of  this  surface  along 
its  dorsolateral  border. 


THE  CEREBELLUM 


199 


2.  The  bivcnlral  lobule,  occupying  the  ventrolateral  part  of  the  inferior  surface. 

3.  The  tonsil,  a  small  rounded  lobule  near  the  inferior  vermis. 

4.  The  flocculus  is  the  smallest  of  the  lobules;  and  from  it  there  runs  toward 
the  median  plane  a  thin  white  band,  the  posterior  medullary  velum,  and  the 
peduncle  of  the  flocculus. 

Structure  of  the  Cerebellum. — The  cerebellum  is  composed  of  a  thin  super- 
ficial lamina  of  gray  matter,  spread  over  an  irregular  white  center  that  con- 
tains several  compact  nuclear  masses.  This  white  medullary  body  forms  a 
compact  mass  in  the  interior  and  is  continuous  from  hemisphere  to  hemisphere 
through  the  vermis,  within  which,  however,  it  is  smaller  than  in  the  hemi- 
spheres (Figs.  140,  141).  As  is  most  readily  seen  in  sagittal  sections  through  the 
cerebellum,  the  medullary  body  gives  off  numerous  thick  laminae,  which  pro- 


Dentaie  nucleus 


Central  lobule 
Lingula 


Culmen 


Fissura  prima 
/  ^ — Declive 


Tuber 
Pyramis 
Nodule         Uvula 
Fig.  140.  Fig.  141. 

Figs.   140  and  141. — Sagittal  sections  of  the  human  cerebellum:    Fig.  140  passes  through  the 
hemisphere  and  dentate  nucleus;  Fig.  141,  through  the  vermis  in  the  median  plane. 

ject  into  the  lobules  of  the  cerebellum;  and  from  these  there  are  given  off  sec- 
ondary and  tertiary  laminae  at  various  angles.  Thus  a  very  irregular  white 
mass  is  formed,  over  the  surface  of  which  the  much  folded  cortex  is  spread  in 
a  thin  but  even  layer.  Supported  by  the  white  laminae,  the  cortex  forms  long 
narrow  folds,  known  as  folia,  which  are  separated  by  sulci  and  which  are  aggre- 
gated into  lobules  that,  in  turn,  are  separated  by  more  or  less  deep  fissures. 
Sections  through  the  cerebellum  at  right  angles  to  the  long  axis  of  the  folia  thus 
present  an  arborescent  appearance  to  which  the  name  arbor  -cita  has  been  ap- 
plied.    This  is  particularly  evident  in  sections  through  the  vermis  (Fig.  141). 

MORPHOLOGY  OF  THE  CEREBELLUM 

According  to  Elliott  Smith  (1903)  and  Bolk  (1906).  who  have  carried  out  extensive 
investigations  on  the  morphology  of  the  mammalian  cerebellum,  the  fissura  prima  is  an 


200 


THE    NERVOUS    SYSTEM 


important  and  constant  fissure.  It  extends  in  a  continuous  curved  line  across  the  rostral 
:  of  the  vermis  and  both  hemispheres.  It  has  been  found  by  Ingvar  (1918)  in  reptiles 
and  birds.  All  investigators  who  have  given  attention  to  this  subject  in  recent  years  agree 
in  designating  the  portion  of  the  cerebellum  which  lies  rostral  to  the  fissura  prima  as  the 
anterior  lobe.  The  portion  behind  this  fissure  is  composed  of  several  individual  lobules,  each 
of  which,  though  subject  to  considerable  variation  in  form  in  the  different  genera,  can  be 
identified  in  every  mammalian  cerebellum.  These  lobules  have  been  variously  grouped  into 
lobes  by  different  invest  igators.  Here  we  will  follow  the  grouping  employed  by  Ingvar,  which 
is  based  on  a  comparison  of  the  mammalian  cerebellum  with  that  of  birds  and  reptiles  (Fig. 
142).  lb-  recognizes  three  major  divisions  of  the  cerebellum,  which  he  designates  as  the 
anterior,  middle,  and  posterior  lobes.  The  middle  lobe  contains  those  parts  of  the  cerebellum 
which  have  been  the  last  to  appear  during  phyletic  development,  and  it  is  here  that  the 
greatesl  variations  are  found  in  the  different  orders  of  mammals. 


1. 


Fig.  142. — Schematic  drawing  of  the  cerebellum  of  1,  lizard;  2,  crocodile;  3,  bird,  and  4, 
mammal.  Vertical  lines,  anterior  lobe;  stipple,  middle  lobe;  horizontal  lines,  posterior  lobe;  white, 
lobus  ansoparamedianus.     (Ingvar.) 


The  anterior  lobe  includes  all  that  part  of  the  cerebellum  that  lies  on  the  rostral  side  of 
the  fissura  prima  (Figs.  143,  144,  146).  In  this  lobe  the  folia  have  a  transverse  direction  and 
extend  without  interruption  across  the  vermis  into  both  hemispheres.  In  the  sheep  the  an- 
terior lobe  is  bounded  laterally  by  the  parafloccular  fissure.  It  includes  the  three  most 
rostral  lobules  of  the  superior  vermis,  which  are  designated  in  order  from  before  backward,  the 
lingula,  lobulus  centralis,  and  admen  monlieuli.  In  man  it  also  includes  a  large  wing-shaped 
portion  of  each  hemisphere  (the  pars  anterior  lobuli  quadrangularis) ;  and  the  entire  lobe  has 
the  shape  of  a  butterfly  (Fig.  146).     Morphologically,  it  is  a  median  unpaired  structure. 

The  middle  lobe  is  subdivided  into  four  parts  (Fig.  142).  The  most  rostral  of  these 
is  the  lobulus  simplex.  It  is  separated  from  the  anterior  lobe  by  the  fissura  prima,  and  like 
that  lobe  it  consists  of  transverse  folia  which  extend  across  the  superior  vermis  into  both 


I  HE    CEREB1  l.l.i  M 


20I 


hemispheres  ( Figs.  143,  144).  In  man  the  lobulus  simplex  forms  a  broad  crescentii  band 
across  the  rostral  surface  of  the  cerebellum,  including  what  is  ordinarily  designated  as  the 
posterior  part  of  the  quadrangular  lobule  and  the  declive  monticuli  (Fig.  146  .  Like  the 
anterior  lobe,  ii  is  a  median  unpaired  structure.  The  remainder  of  the  middle  lobe  is  sub- 
divided into  median  and  lateral  portions.     The  median  part,  known  as  the  tuber  :*r»iis 


Fissura  prima 


Lobulus  ansiformis 


Lobulus  paramedianus  *— 


LobuS  anleriiir 

ft  Lobulus  simplex 

.-  Parafloi  <  ulus 

Fissura  Parafloct  ularis 


—^Tuber  vermis 


Fig.  143. — Cerebellum  of  the  sheep,  dorsorostral  view. 

(lobulus  medius  medianus  of  Ingvar  and  lobulus  C2  of  Bolk),  forms  a  conspicuous  S-shaped 
lobule  in  I  he  vermis  of  the  sheep  (Fig.  145)  and  may  be  readily  identified  at  the  occipital 
extremity  of  the  inferior  vermis  in  man  (Figs.  139,  141).  The  paired  lateral  portions  of  the 
middle  lobe  each  consist  of  two  parts,  called  the  lobulus  ansiformis  and  lobulus  paramedianus. 
The  lobulus  ansiformis,  relatively  small  in  most  mammals  (Fig.  144),  is  very  large  in  man, 


Fissura  prima 
i 

,  Lobus  anterior 

,Lobulus  simplex 


Flocculus-"  ~\lfS 

Parajlocculus  *" 


Lobulus  paramedianus '' 


Lobulus  ansiformis 

-~a"  Tuber  vermis 

Lobulus  medianus  posterior 


Fig.  144.— Cerebellum  of  the  sheep,  lateral  view. 

and  forms  approximately  the  dorsolateral  half  of  the  hemisphere,  occupying  considerable 
parts  of  both  the  rostral  and  caudal  surfaces.  It  corresponds  to  what  has  been  known  as 
the  superior  and  inferior  semilunar  lobules  and  the  biventral  lobule  (Figs.  146,  147).  The 
lobulus  paramedianus,  or  tonsilla  of  the  B.  N.  A.,  is  located  on  the  lateral  surface  of  the 
sheep's  cerebellum,  but  is  displaced  on  to  the  caudal  surface  in  man  by  the  great  expansion 
of  the  lobulus  ansiformis. 


202  THE   NERVOUS    SYSTEM 

The  posterior  lobe,  as  outlined  by  Ingvar,  is  composed  of  median  and  lateral  portions. 
The  median  part,  known  as  the  posterior  median  lobule,  comprises  all  of  the  inferior  vermis 
except  the  tuber,  from  which  it  is  separated  by  the  prepyramidal  sulcus.  It  is  subdivided 
into  three  sublobules,  known  as  the  nodule,  uvula,  and  pyramid  (Figs.  139,  141,  145).  The 
lateral  part  of  the  posterior  lobe  is  formed  on  either  side  by  two  lobules,  known  as  the  flocculus 
and  paraflocculus.  These  form  the  most  lateral  portion  of  the  hemisphere  in  most  mammals 
(Figs.  142,  144).  In  man  the  paraflocculus  is  rudimentary  and  the  flocculus  lies  upon  the 
caudal  surface  of  the  hemispheres  (Fig.  147).  It  is  connected  with  the  nodule  by  a  thin 
sheet  of  white  matter,  the  posterior  medullary  velum. 

Functional  Localization  in  the  Cerebellum. — We  have  described  the  cerebellum  in 
terms  of  the  subdivisions  of  Bolk  and  Ingvar,  because  these  have  morphologic  and  physio- 
logic significance,  which  is  not  true  of  the  parts  into  which  the  cerebellum  had  previously 
been  divided.  By  comparison  of  the  size  of  these  subdivisions  with  the  degree  of  develop- 
ment and  functional  importance  of  the  various  groups  of  muscles  in  different  animals  Bolk 
endeavored  to  show  that  each  of  these  parts  was  related  to  a  particular  group  of  muscles. 
On  the  basis  of  these  comparative  studies  he  concluded  that  the  median  unpaired  portions 
of  the  cerebellum  serve  as  coordination  centers  for  the  muscles  which  function  in  bilateral 

Tuber  vermis 


Prepyramidal  sulcus v       ^--j^k  y^^  ^c^^  ,tui  •/ 

tJ  ^jC-  r~*J?ix      ■- "  x§c\  -"' Lobulus  ansijormis 


Paraflocculus-- --^a^    ^^%^C^^^-~^^rY0r^^-^t — Lobulus  paramedianus 


~"  Lob  id  us  medianus  posterior 
/ 


Fig.  145. — Cerebellum  of  the  sheep,  caudal  view. 

synergy.  The  muscles  of  expression  and  mastication,  those  of  the  eyes,  pharynx,  larynx 
and  neck,  and  many  of  the  trunk  muscles  are  called  into  action  simultaneously  on  both  sides 
of  the  body,  and  should,  according  to  this  theory,  have  a  median  unpaired  representation 
in  the  cerebellum.  Bolk  located  the  coordination  center  for  the  musculature  of  the  head 
in  the  anterior  lobe,  that  for  the  muscles  of  the  neck  in  the  lobulus  simplex  (Figs.  146,  147). 
A  median  center  for  those  movements  of  the  extremities  which  are  strictly  bilateral  is  found 
in  the  most  dorsal  sublobule  of  the  vermis  inferior,  known  as  lobulus  C"2  or  tuber  vermis. 
The  remainder  of  the  inferior  vermis  forms,  according  to  this  theory,  a  center  for  the  bilateral 
movements  of  the  trunk.  In  addition  to  a  median  center  in  the  tuber  vermis,  the  limbs  are 
represented  in  the  cerebellum  by  lateral  centers  for  the  coordination  of  unilateral  move- 
ments. The  lateral  center  for  the  arm  is  located  in  the  rostral  part  or  crus  primum  of  the 
lobulus  ansiformis  (superior  and  inferior  semilunar  lobules)  and  that  for  the  legs  in  the  caudal 
part  or  crus  secundum  (biventral  lobule),  and  perhaps  also  in  the  lobulus  paramedianus 
(tonsil). 

The  conclusions  concerning  the  localization  of  function  in  the  cerebellum,  reached  by 
Bolk  on  the  basis  of  morphologic  studies,  have  been  confirmed  in  so  far  as  the  centers  for  the 
neck  and  extremities  are  concerned  by  animal  experimentation  (Van  Rynberk,  1908,  1912; 


Till.    <  I  REBELLUM 


203 


Andre  Thomas  and  Durupt,  1914)  and  by  clinical  observations  (B&rany,  1'>1_').  There 
are,  however,  good  reasons  for  skepticism  regarding  his  localization  ol  centers  for  the  head 
and  trunk,  [ngvar  (1918)  presents  evidence  which  indicates  thai  the  anterior  and  posterior 
lobes  arc  probably  concerned  with  the  maintenance  of  the  equilibrium  of  the  bod)  as  a  whole. 
I  he  middle  lobe,  on  t he  ol her  hand,  contains  a  number  of  separate  <  enters,  wbi<  h  correspond 
to  those  out  lined  by  Hoik,  for  t  he  eont  rol  of  i  he  rnus<  ulat  ure  of  the  neck  and  extremit 

It  has  long  been  known  thai  the  degree  of  development  of  the  cerebellar  hemispheres  in  the 
different  classes  of  vertebrates  i-*  closely  <  orrelated  with  that  of  the  pons  and  c  erebral  1  ortex. 
Tin's  is  particularly  true  of  the  lobulus  ansiformis  and  lobulus  paramedianus,  win.  h,  like  the 
aeopallium,  are  recent  phyletic  developments.     These  belong  to  what  Edinger  (1911)  calls 


/:    N.  A. 

Ala  liilmli  centralis 

Lobulus  centralis 

Culmen  monticuli 

J'ars  anterior  lobuli 

quadrangularis 

Pars  posterior  lobuli 

quadrangularis 

Declive  montu  uli 

Lobulus  semilunaris 
superior 


Fig.  146. 


Lobulus  centralis 

A 'a  lobuli  centra' is 

Brae liiuni  pontis 

Flocculus 

Brae  hi  urn  conjunct  ivum 

Nodulus 

I  'vula 

'J' on  si' I  a 

Lobulus  biventer 

Pyramis 

Tuber 

Lob.  semilun.  inf. 

Sulcus  horizontalis 

Lobulus  semilunaris 

superior 

Figs.  146  and  147. — Outline  drawings  of  the  human 
function  according  to  the  theory  of  Bolk.     On  the  right 
to  Bolk's  terminology,  on  the  left  according  to  the  B.  X 
ventral  view.     (Herrick.) 


BOLR 

Lobui  anterior 


Sulcus  pri mar ius 
Lobulus  simplex 
S.  postt  livalis 

Lobulus  ansiformis 


f.obus  anterior 

Cerebellar  peduncles  (cut) 
Flocculus 

Sulcus  uvulo-nodularis 

Lobulus  paramedianus 

Fissura  sccunda 


Lobulus  ansiformis 


cerebellum  showing  the  localization  of 

side  the  parts  are  designated  according 

A.     Fig.  146,  dorsal  view.     Fig.  147, 


the  neocerebellum,  receive  the  majority  of  the  fibers  from  the  brachium  pontis,  and  may 
properly  be  regarded  as  cortical  dependencies.  They  take  an  important  part  in  the  co- 
ordination of  the  voluntary  movements  of  the  extremities. 


THE  NUCLEI  OF  THE  CEREBELLUM 
The  dentate  nucleus  is  a  crumpled,  purse-like  lamina  of  gray  matter  within 
the  massive  medullary  body  of  each  cerebellar  hemisphere  (Fig.  148).  Like 
the  inferior  olivary  nucleus,  which  it  closely  resembles,  it  has  a  white  center 
and  a  medially  placed  hilus.  In  close  relation  to  this  hilus  lies  a  plate  of  gray 
matter,  the  cmboliform  nucleus,  and  medial  to  this  is  the  small  globose  nucleus. 


204 


THE    NERVOUS    SYSTEM 


Close  to  the  median  plane  in  the  medullary  body  of  the  vermis,  where  this  forms 
the  tent-like  covering  of  the  fourth  ventricle,  is  the  nucleus  of  the  roof  or  nucleus 

fastigii. 

The  dentate  nucleus  is  well  developed  only  in  those  animals  which  possess 
large  cerebellar  hemispheres.  It  receives  fibers  from  the  cortex  of  the  cere- 
bellar hemisphere,  while  the  nuclei  fastigii  and  globosi  receive  fibers  chiefly 
from  the  vermis  (Clark  and  Horsley,  1905;  Edinger,  1911).     It  is  probable  that 

Rhomboid  fossa 

Decussation  of  brachia  conjunctiva  --,.-• 
Medial  longitudinal  fasciculus"  /  A  ^rior  medullary  velum 


Brachium  conjunctivum  s 

Molecular 
Granular  layer 


Lingula  of  cerebellum 
Fastigial  nucleus 

llilus  of  dentate  nucleus 
Dentate  nucleus 


Medullary  la  miner'  \fr^  <\/t\K£ 

Cerebellar  folia^'-*^ .. 

Medullary  substance  of  -•' 
hemisphere 

Emboliform  nucleu 

Globose  nucleus 


Vermis 


Capsule  of  dentate  nucleus 
Posterior  cerebellar  notch 


Fig.  148. — Horizontal  section  through  the  cerebellum  showing  the  location  of  the  central  nuclei. 

(Sobotta-McMurrich.) 

a  functional  localization  similar  to  that  in  the  cerebellar  cortex  will  be  found 
to  exist  in  the  central  nuclei.  In  histologic  structure  the  central  nuclei  closely 
resemble  the  inferior  olive. 


THE  CEREBELLAR  PEDUNCLES 

The  white  core  of  the  cerebellum  is  formed  in  large  part  of  fibers  which  enter 
and  leave  the  cerebellum  through  its  three  peduncles. 

The  brachium  pontis,  or  middle  cerebellar  peduncle,  is  formed  by  the  trans- 
verse fibers  of  the  pons  and  carries  impulses  which  come  from  the  cerebral  cortex 
of  the  opposite  side.  It  enters  the  cerebellum  on  the  lateral  side  of  the  other 
two,  and  is  distributed  in  two  great  bundles:  one  from  the  rostral  part  of  the 
pons  radiates  to  the  caudal  part  of  the  cerebellar  hemisphere;  the  other,  from  the 
caudal  part  of  the  pons,  spreads  out  to  the  rostral  portion  of  the  hemisphere. 
In  man,  as  might  be  expected  from  the  large  size  of  the  pons  and  cerebellar 


T1IK    CKRKHKLLUM 


205 


hemispheres,  the  brachium  pontis  is  the  largesl  of  the  three  peduncles  (Fig. 
89).  lUit  this  is  not  true  in  most  mammals,  where,  as  in  the  sheep,  the  cere- 
bellum receives  the  majority  of  its  afferent  fibers  from  the  spinal  cord  and  medulla 
oblongata  by  way  of  the  relatively  large  restiform  bodies  (Fig.  91). 

The  restiform  body  ascends  along  the  lateral  border  of  the  fourth  ventricle; 
and  at  a  point  jusl  rostral  to  the  lateral  recess  it  makes  a  sharp  turn  dorsally 
to  enter  the  cerebellum  between  the  other  two  peduncles  (Figs.  87,  88).  It 
consists  of  ascending  fibers  from  the  spinal  cord  and  medulla  oblongata  and  prob- 
ably also  of  descending  fibers  from  the  cerebellum  to  the  reticular  formation 
of  the  medulla  ( fastigiobulbar  tract,  p.  211).  Among  the  ascending  fibers  are 
those  of  the  following    bundles:  (1)   dorsal   spinocerebellar   tract,  which   arises 


Res 

Dorsal  spinoco 
Ventral  spinocc 


Tcctoccrebellar  trad 


'         -/  Corpora  quadrigemina 


■  Brachium  con junctivum 


Pons 


Fig.  149. — Diagram  of  the  spinocerebellar  and  tectocerebellar  tracts. 


from  the  cells  of  the  nucleus  dorsalis  of  the  same  side  of  the  spinal  cord  and 
ends  in  the  cortex  of  the  vermis;  (2)  the  olivocerebellar  tract,  which  consists  of 
fibers  from  the  opposite  inferior  olivary  nucleus  and  to  a  less  extent  from  that 
of  the  same  side  and  which  ends  in  the  cortex  of  the  vermis  and  of  the  hemi- 
sphere and  in  the  central  nuclei;  (3)  the  dorsal  external  arcuate  fibers,  from  the 
nuclei  of  the  posterior  funiculi  of  the  same  side;  (4)  ventral  external  arcuate 
fibers  from  the  arcuate  and  lateral  reticular  nuclei  (Fig.  104). 

The  so-called  medial  part  of  the  restiform  body  consists  of  bundles  of  fibers 
belonging  to  the  tractus  nucleocercbcllaris,  which  course  along  the  medial  side  of 
that  peduncle  as  it  turns  dorsally  into  the  cerebellum  (Fig.  110).  These  come 
from  the  sensory  nuclei  of  the  cranial  nerves.  Most  of  them  arise  from  the 
superior  and  lateral  vestibular  nuclei  or  represent  the  ascending  branches  of  the 


206  THE   NERVOUS    SYSTEM 

fibers  of  the  vestibular  nerve  and  constitute  the  tractus  vestibulocerebellaris. 
According  to  Cajal  (1911)  the  fibers  of  this  tract  are  distributed  to  the  cortex 
of  the  cerebellum,  the  majority  of  them  going  to  the  vermis,  a  smaller  proportion 
to  hemisphere.  In  view  of  the  newer  ideas  concerning  the  morphology  of  the 
cerebellum,  the  statements  concerning  the  termination  of  all  these  cerebellar 
afferent  fibers  require  re-examination. 

The  brachium  conjunctivum  (Fig.  88)  consists  of  efferent  fibers  from  the 
dentate  nucleus  to  the  red  nucleus  and  the  thalamus  of  the  opposite  side.  It  is 
the  smallest  and  most  medial  of  the  three  peduncles.  The  ventral  spinocere- 
bellar tract  enters  the  cerebellum  in  company  with  the  brachium  conjunctivum. 
It  ascends  through  the  medulla  oblongata  and  pons,  curves  over  the  brachium 
conjunctivum  (Fig.  110),  and  enters  the  anterior  medullary  velum,  within  which 
it  runs  to  the  cerebellum  (Fig.  149).  Its  fibers  terminate  in  the  rostral  part  of 
the  vermis  and  in  the  nucleus  fastigii  (Fforrax,  1915).  According  to  Edinger, 
a  bundle  of  fibers,  the  tcctocerebellar  tract,  arises  in  the  tectum  of  the  mesencepha- 
lon and  descends  alongside  of  the  brachium  conjunctivum  to  the  cerebellum, 
probably  conveying  impulses  from  visual  centers. 

According  to  MacNalty  and  Horsley  (1909)  and  Ingvar  (1918)  the  fibers  of  the  ventral 
spinocerebellar  tract  end  in  the  lobulus  centralis,  culmen,  and  most  rostral  part  of  the  declive. 
The  fibers  of  the  dorsal  spinocerebellar  tract  have  the  same  termination  and,  in  addition, 
many  of  them  go  to  the  pyramis,  and  smaller  numbers  to  the  uvula  and  nodule.  Practically 
all  of  the  fibers  which  end  in  the  cortex,  therefore,  go  to  the  anterior  and  posterior  lobes 
(Ingvar).  The  fact  that  the  anterior  lobe  receives  the  majority  of  these  fibers,  which  convey 
proprioceptive  impulses  from  the  trunk  and  extremities,  is  a  strong  argument  against  Bolk's 
conception  of  the  anterior  lobe  as  a  co-ordination  center  for  the  musculature  of  the  head. 

HISTOLOGY  OF  THE  CEREBELLAR  CORTEX 

The  cerebellar  cortex  differs  from  that  of  the  cerebral  hemispheres  in  pos- 
sessing essentially  the  same  structure  in  all  the  lobules.  This  would  indicate 
that  it  functions  in  essentially  the  same  way  throughout,  though  as  a  result  of 
different  fiber  connections  the  various  lobules  act  on  different  muscle  groups. 

A  section  through  the  cerebellum,  taken  at  right  angles  to  the  long  axis 
of  the  folia,  shows  each  folium  to  be  composed  of  a  central  white  lamina,  covered 
by  a  layer  of  gray  cortex.  Within  the  white  lamina  the  nerve-fibers  are  arranged 
in  parallel  bundles  extending  from  the  medullary  center  of  the  cerebellum  into 
the  lobules  and  folia.  A  few  at  a  time  these  bundles  turn  off  obliquely  into  the 
gray  matter,  and  there  is  no  sharp  demarcation  between  the  cortex  and  the  sub- 
jacent white  lamina.     The  cortex  presents  for  examination  three  well-defined 


THE    CEREBELLUM 


207 


zones:  a  superficial  molecular  layer,  a  layer  of  Purkinje  cells,  and  a  subjacent 
granular  layer. 

The  cells  of  Purkinje  have  large  flask-shaped  bodies  and  are  arranged  in  an 
almost  continuous  sheet,  consisting  of  a  single  layer  of  cells  and  separating 
the  other  two  cortical  zones  (Fig.  150).  They  are  more  numerous  at  the  summit 
than  at  the  base  of  the  folium.  Each  has  a  pyriform  cell  body.  The  part 
directed  toward  the  surface  of  the  cortex  resembles  the  neck  of  a  flask  and  from 


Fig.  150. — Semidiagrammatic  transverse  section  through  a  folium  of  the  cerebellum.  (Golgi 
method):  A,  Molecular  layer;  B,  granular  layer;  C,  white  matter;  a,  Purkinje  cell;  b,  basket  cells; 
d,  pericellular  baskets,  surrounding  the  Purkinje  cells  and  formed  by  the  arborizatiens  of  the 
axons  of  the  basket  cells;  e,  superficial  stellate  cells;/,  cell  of  Golgi  Type  II;  g,  granules,  whose 
axons  enter  the  molecular  layer  and  bifurcate  at  i\  h,  mossy  fibers;.;  and  m,  neuroglia;  n,  climb- 
ing fibers.     (Cajal.) 


it  spring  one  or  two  stout  dendrites.  These  run  into  the  molecular  layer  and 
extend  throughout  its  entire  thickness,  branching  repeatedly.  This  branching 
occurs  in  a  plane  at  right  angles  to  the  long  axis  of  the  folium;  and  it  is  only  in 
sections,  taken  in  this  plane,  that  the  full  extent  of  the  branching  can  be  ob- 
served. In  a  plane  corresponding  to  the  long  axis  of  the  folium  the  dendrites 
occupy  a  more  restricted  area  (Fig.  151).  In  this  respect  the  dendritic  ramifica- 
tions resemble  the  branches  of  a  vine  on  a  trellis.  From  the  larger  end  of  the 
cell,  directed  away  from  the  surface  of  the  cortex,  there  arises  an  axon  which 


208 


THE    NERVOUS    SYSTEM 


almost  at  once  becomes  myelinated  and  runs  through  the  granular  layer  into  the 
white  substance  of  the  cerebellum.  According  to  Clarke  and  Horsley  (1905)  and 
Cajal  (1911)  these  axons  end  in  the  central  cerebellar  nuclei.  Near  their  origin 
they  give  off  collaterals,  which  run  backward  through  the  molecular  layer  to 
end  in  connection  with  neighboring  Purkinje  cells — an  arrangement  designed 
to  bring  about  the  simultaneous  discharge  of  a  whole  group  of  such  neurons. 
The  granular  layer,  situated  immediately  subjacent  to  that  which  we  have 
just  described,  is  characterized  by  the  presence  of  great  numbers  of  small  neurons, 
the  granule  cells.  Each  of  these  contains  a  relatively  large  nucleus,  surrounded 
by  a  small  amount  of  cytoplasm;  and  from  each  there  are  given  off  from  three 
to  five  short  dendritic  branches  with  claw-like  endings.  These  are  synaptically 
related  with  the  terminal  branches  of  the  moss  fibers,  soon  to  be  described,  and 


Purkinje  cell" 

Basket  cell" 

Granule  cell" 


"Purkinje  cell 
"~  Granule  cell 


Fig.  151. — Diagrammatic  representation  of  the  structure  of  the  cerebellar  cortex  as  seen 
in  a  section  along  the  axis  of  the  folium  (on  the  right),  and  in  a  section  at  right  angles  to  the  axis 
of  the  folium  (on  the  left). 


form  with  them  small  glomeruli  comparable  to  those  of  the  olfactory  bulb  (Fig. 
208).  Each  granule  cell  gives  origin  to  an  unmyelinated  axon,  which  extends 
toward  the  surface  of  the  folium  and  enters  the  molecular  layer.  Here  it  divides 
in  the  manner  of  a  T  into  two  branches.  These  run  parallel  to  the  long  axis  of 
the  folium  through  layer  after  layer  of  the  dendritic  expansions  of  the  Purkinje 
cells,  with  which  they  doubtless  establish  synaptic  relations  (Fig.  151).  Besides 
the  granules  just  described,  this  layer  contains  some  large  cells  of  Golgi's  Type 
II  (Fig.  150,  /).  Most  of  these  are  placed  near  the  line  of  Purkinje  cells  and 
send  their  dendrites  into  the  molecular  layer,  while  their  short  axons  resolve 
themselves  into  plexuses  of  fine  branches  in  the  granular  zone. 

The  molecular  layer  contains  few  nerve-cells  and  has  in  transverse  sections 
a  finely  punctate  appearance.  It  is  composed  in  large  part  of  the  dendritic 
ramifications  of  the  Purkinje  cells  and  the  branches  of  axons  from  the  granule 


THE    CEREBELLUM 


209 


cells  (Fig.  150).  It  contains  a  relatively  small  number  of  stellate  neurons,  the 
more  superficial  of  which  possess  short  axons  and  belong  to  Golgi's  Type  II. 
Those  more  deeply  situated  have  a  highly  specialized  form  and  arc  known  as 
basket  cells.  From  each  of  these  there  arises,  in  addition  to  several  stout  brandl- 
ing dendrites,  a  single  characteristic  axon,  which  runs  through  the  molecular 
layer  in  a  plane  at  right  angles  to  the  long  axis  of  the  folium  (Fig.  151).  These 
axons  are  at  first  very  fine,  but  soon  become  coarse  and  irregular,  giving  off 
numerous  collaterals  which  are  directed  away  from  the  surface  of  the  cortex. 
These  collaterals  and  the  terminal  branches  of  the  axons  run  toward  the  Purkinje 
cells,  about  which  their  terminal  arborizations  form  basket-like  networks  (Fig.  29). 


Purkinje  < 
Dentate  nucleus 

Brachium  conjunc- 

tivum 


Brachium  pontis 


Restiform  body 

Climbing  fibers'' 
Mossy  fibers  - 


Basket  cell 
"  Granule  cell 


Fig.  152. — Diagram  to  illustrate  the  probable  lines  of  conduction  through  the  cerebellum. 

Nerve-fibers. — The  axons  of  the  Purkinje  cells  form  a  considerable  volume 
of  fibers  directed  away  from  the  cortex.  There  are  also  two  kinds  of  afferent 
fibers  which  enter  the  cortex  from  the  white  center,  and  are  known  as  climbing 
and  mossy  fibers  respectively.  The  latter  are  very  coarse  and  give  off  numerous 
branches  ending  within  the  granular  layer.  The  terminal  branches  are  provided 
with  characteristic  moss-like  appendages.  These  mossy  tufts  are  intimately 
related  to  the  claw-like  dendritic  ramifications  of  the  granule  cells  (Fig.  152). 
The  climbing  fibers,  somewhat  finer  than  those  of  the  preceding  group,  pass 
through  the  molecular  layer  and  become  associated  with  the  dendrites  of  the 
Purkinje  cells  in  the  manner  of  a  climbing  vine.     Branching  repeatedly,  they 


2IO  THE    NERVOUS    SYSTEM 

follow  closely  the  dendritic  ramifications  of  these  neurons  and  terminate  in  free 
varicose  endings. 

It  would  seem  reasonable  to  suppose  that  the  two  kinds  of  afferent  fibers, 
just  described,  have  a  separate  origin  and  functional  significance.  According 
to  Cajal  (1911)  it  is  probable  that  those  entering  the  cerebellum  through  the 
brachium  pontis  are  distributed  as  climbing  fibers,  and  those  from  the  restiform 
body  as  mossy  fibers.  The  accompanying  diagram  represents  the  probable 
course  of  impulses  through  the  cerebellum  (Fig.  152).  The  mossy  fibers,  prob- 
ably derived  from  the  restiform  body,  transfer  their  impulses  to  the  granule 
cells;  and  these,  in  turn,  relay  them,  either  directly  or  through  the  basket  neu- 
rons, to  the  Purkinje  cells.  The  climbing  fibers,  which  probably  come  from  the 
brachium  pontis,  transfer  their  impulses  directly  to  the  dendrites  of  the  Purk- 
inje cells.  We  do  not  known  to  which  class  the  fibers  of  the  vestibulocerebellar 
tract  should  be  assigned.  The  efferent  path  may  be  said  to  begin  with  the 
Purkinje  cells,  whose  axons  terminate  in  the  central  cerebellar  nuclei.  From 
these  nuclei,  especially  the  dentate,  arise  the  fibers  of  the  brachium  conjunc- 
tivum,  the  great  efferent  tract  from  the  cerebellum.  By  means  of  the  axons 
of  the  granule  cells,  basket  cells,  and  neurons  of  Golgi's  Type  II,  as  well  as  by 
the  collaterals  from  the  axons  of  the  Purkinje  cells,  an  incoming  impulse  may  be 
diffused  through  the  cortex. 

The  cerebellum  probably  receives  fibers  from  all  the  somatic  sensory  centers, 
but  especially  from  those  o*  the  pioprioceptive  group,  through  which  afferent 
impulses  are  err  eyed  to  it  from  the  muscles,  joints  and  tendons,  and  from 
the  semicircular  canals  of  the  ear.  Its  connection  with  the  vestibular  appa- 
ratus is  especially  intimate.  In  fact,  as  already  stated,  it  may  be  regarded  from 
the  standpoint  of  development  as  a  very  highly  specialized  portion  of  the  ves- 
tibular nucleus.  It  is  the  great  proprioceptive  correlation  center.  Further- 
more, it  sends  efferent  impulses  to  the  various  somatic  motor  centers  and  plays 
an  important  part  in  the  coordination  of  muscular  contraction  and  in  the  main- 
tenance of  muscular  tone.  It  is  the  chief  center  for  equilibration,  which  depends 
upon  the  proper  adjustment  of  the  muscles  in  response,  very  largely,  to  the 
impulses  from  the  semicircular  canals.  In  man  and  mammals  it  also  receives 
impulses  from  the  cerebral  cortex  by  way  of  the  pons,  which  probably  set  the 
coordinating  cerebellar  mechanism  into  activity  to  bring  about  the  proper 
adjustment  of  voluntary  movements.  For  additional  details  concerning  the 
functions  of  the  cerebellum  the  reader  should  consult  the  recent  paper  by 
Holmes  (1917). 


THE    CKKKHKLLl.'M 


211 


THE  EFFERENT  CEREBELLAR  TRACTS 

The  efferent  cerebellar  tracts  arise  in  the  central  nuclei.  It  is  probable  that 
no  fibers  of  cortical  origin  leave  the  cerebellum  except,  perhaps,  some  to  Deiter's 
nucleus  (Clarke  and  Horsley,  1905). 

The  brachium  conjunctivum,  or  tractus  cerebellotegmentalis  mesencephali, 
arises  for  the  most  part  at  least  in  the  dentate  nucleus  and  terminates  in  the  red 
nucleus  and  thalamus  of  the  opposite  side  (Fig.  153).  It  constitutes  the  chief 
tract  leading  from  the  cerebellum  and  has  been  more  fully  described  on  page 
159.  It  undergoes  a  complete  decussation  beneath  the  inferior  colliculus  in 
the  tegmentum  of  the  mesencephalon.     Both  before  and  after  this  crossing  its 


Brachium  conj 


Fastigiobulbar  tract 


Thalamus 

Red  nucleus 
y  Nucleus  fastigii 

-  Nucleus  dentatus 


--Tractus  cerebellotegmentalis 

pontis 
"' 'Lateral  vestibular  nucleus 

a  ..'.'  ■'obulbar  tract 


Fig.  153. — Efferent  tracts  which  arise  in  the  central  nuclei  of  the  cerebellum.     (Modified  from 

Edinger.) 


fibers  give  off  branches,  which  descend  in  the  reticular  formation  of  the  pons 
and  medulla.  Some  of  the  impulses  reach  the  thalamus,  but  the  others  are 
relayed  in  the  red  nucleus  along  the  rubrospinal  and  rubroreticular  tracts  to 
motor  neurons  in  the  brain  stem  and  spinal  cord  (Fig.  115). 

Other  efferent  tracts  arise  in  the  nucleus  fastigii  of  the  same  and  opposite 
side,  and  run,  probably  by  way  of  all  three  cerebellar  peduncles,  to  the  retic- 
ular formation  of  the  pons  and  medulla  oblongata.  One  bundle  of  these  fibers 
winds  around  the  brachium  conjunctivum  before  descending  through  the  pons 
and  medulla  (Fig.  153).  It  is  probable  that  other  fibers  descend  by  way  of  the 
restiform  body,  and  are  distributed  in  the  reticular  formation  of  the  medulla 


212  THE    NERVOUS   SYSTEM 

oblongata  on  the  same  side,  or  are  continued  as  ventral  external  arcuate  fibers 
to  end  on  the  opposite  side.  The  bundles  which  run  from  the  nucleus  fastigii 
to  the  medulla  oblongata  may  be  designated  as  the  fastigiobulbar  tracts  (tractus 
cerebellotegmentales  bulbi).  These  include  fibers  which  terminate  in  the 
lateral  vestibular  nucleus.  It  is  said  that  some  fibers  belonging  to  this  system 
leave  the  cerebellum  by  way  of  the  brachium  pontis  (tractus  cerebellotegmentalis 
pontis) . 

Since  the  dentate  nucleus  receives  fibers  from  the  cortex  of  the  correspond- 
ing cerebellar  hemisphere,  and  the  nucleus  fastigii  receives  similar  fibers  from 
the  vermis,  it  may  be  inferred  that  the  brachium  conjunctivum  is  the  chief 
efferent  tract  for  the  hemisphere  and  that  the  fastigiobulbar  tracts  serve  the 
same  purpose  for  the  vermis  (Strong,  1915). 


CHAPTER  XIV 

THE  DIENCEPHALON  AND  THE  OPTIC  NERVE 

Development. — In  an  earlier  chapter  we  traced  briefly  the  development  of 
the  prosencephalon  and  showed  that  the  cerebral  hemispheres  were  developed 
through  the  evagination  of  the  lateral  walls  of  the  telencephalon  (Fig.  16).     It 

is,  however,  only  the  alar  lamina  which  is  involved  in  this  evagination.  The 
basal  lamina  of  the  telencephalon  retains  its  primitive  position  and  forms  the 
pars  optica  hypothalami.  This  part  of  the  hypothalamus,  along  with  the 
lamina  tcrminalis  and  the  most  rostral  part  of  the  third  ventricle,  constitutes 
the  telencephalon  medium  (Johnston,  1912).  Through  the  excessive  growth  of 
the  hemisphere  the  diencephalon  becomes  covered  from  view  (Fig.  17),  and 
appears  to  occupy  a  central  position  in  the  adult  human  brain.  It  is  separated 
from  the  hemisphere  by  the  transverse  cerebral  fissure,  which  is  formed  by  the 
folding  back  of  the  hemisphere  over  the  diencephalon.  The  differentiation  of 
the  alar  lamina  of  the  diencephalon  into  the  thalamus,  epithalamus,  and  meta- 
t  ha  lam  us,  and  of  its  basal  lamina  into  the  hypothalamus  was  briefly  traced  on 
page  34.  The  sulcus  limitans,  which  separates  these  two  plates  in  the  embryo, 
corresponds  to  the  more  caudal  portion  of  the  hypothalamic  sulcus  of  the  adult; 
but,  since  the  latter  can  be  followed  to  the  interventricular  foramen,  while  the 
former  ends  near  the  optic  chiasma,  the  rostral  ends  of  these  two  sulci  are  not 
related.  The  roof  plate  of  the  prosencephalon  remains  thin  and  constitutes 
the  epithelial  roof  of  the  third  ventricle,  which  along  the  median  plane  becomes 
invaginated  into  the  ventricle  as  the  covering  of  a  vascular  network  to  form 
the  chorioid  plexus. 

THE  THALAMUS 

The  thalamus  is  a  large  ovoid  mass,  consisting  chiefly  of  gray  matter,  placed 
obliquely  across  the  rostral  end  of  the  cerebral  peduncle  (Figs.  154,  155).  Be- 
tween the  two  thalami  a  deep  median  cleft  is  formed  by  the  third  ventricle. 
The  rostral  end  is  small  and  lies  close  to  the  median  plane.  It  projects  slightly 
above  the  rest  of  the  dorsal  surface,  forming  the  anterior  tubercle  of  the  thalamus, 
and  helps  to  bound  the  interventricular  foramen  (Fig.  154).  The  caudal  ex- 
tremity is  larger  and  is  separated  from  its  fellow  by  a  wide  interval,  in  which  the 


214 


THE    NERVOUS    SYSTEM 


corpora  quadrigemina  appear.  It  forms  a  marked  projection,  the  pulvinar, 
which  overhangs  the  medial  geniculate  body  and  the  brachia  of  the  corpora 
quadrigemina  (Figs.  88,  154).  For  purposes  of  description  it  is  convenient  to 
recognize  four  thalamic  surfaces,  namely,  dorsal,  ventral,  medial,  and  lateral. 

The  dorsal  surface  of  the  thalamus  is  free  (Figs.  91,  154).  It  forms  the 
floor  of  the  transverse  fissure  of  the  cerebrum  and  is  separated  by  this  fissure 
from  the  parts  of  the  cerebral  hemisphere  which  overlie  it,  that  is,  from  the 


Free  portions  of  columns  of  fornix. 
Head  of  caudate  nucleus^ 
Medullary  stria,  s  '"• 
Third  ventricle  > 

Habenular  trigone  v       \ 


Pineal  body~j 
Superior  colliculus \] 


Tail  of  caudate  nucleus- 


Super,  quadrigeminal  brack. 
Infer,  quadrigeminal  brack. 
Cerebral  peduncle 
Corpora  quadrigemina 
Lateral  filaments  of  pons 
Anterior  medullary  velum 


Lingula  of  cerebellum 


Tela  chorioidea  of  fourth  ventricle 


Corpus  callosum 

Lamina  of  septum  pdlucidum 
/  Columns  of  fornix 

y  A  ntcrior  commissure 

'  y  Optic  recess  of  ventricle  III 

■'  •'«  -  A  ntcrior  tubercle  of  thalamus 

''  y  Terminal  stria 

s  ^,'Tcenia  chorioidea 

■  Habenular  commissure 
Lamina  affixa 
Superior  quadrigeminal 
brachium 

.-'Pulvinar  of  thalamus 

^Lateral  geniculate  body 


Medial  geniculate  body 
Inferior  colliculus 
Trochlear  nerve 
.Brachium  conjuctivum 

Lateral  recess  of  fourth  ventricle 
'  Brachium  pontis 
.Peduncle  of  flocculus 


Flocculus  of  cerebellum 

Lateral  aperture  of  ventricle  IV 
iChorioid  plexus  of  ventricle  IV 
(Rhomboid  fossa  (intermediate  portion) 
Medial  aperture  of  ventricle  IV 

Funiculus  gracilis 


Medulla  oblongata 
Fig.  154.— Dorsal  view  of  the  human  brain  stem.     (Sobotta-McMurrich.) 

fornix  and  corpus  callosum.  Laterally  it  is  bounded  by  a  groove,  which  separates 
it  from  the  caudate  nucleus  and  contains  a  strand  of  longitudinal  fibers,  the 
stria  terminalis  and  a  vein,  the  vena  terminalis  (Figs.  154,  155).  The  dorsal 
surface  is  separated  from  the  medial  by  a  sharp  ridge,  the  tcenia  thalami,  which 
represents  the  torn  edge  of  the  ependymal  roof  of  the  third  ventricle.  The 
taeniae  of  the  two  sides  meet  in  the  stalk  of  the  pineal  body.  The  prominence 
of  this  torn  edge  of  the  roof  is  increased  by  a  longitudinal  bundle  of  fibers, 


THE    DIENCEPHALON    AND    Till!    (tl'TIC    NERVE 


2I5 


the  stria  medullaris  thalami.  This  fascicle,  together  with  the  closely  related 
habenular  trigone  and  the  pineal  body,  belong  to  the  epithalamus  and  will  be 
described  later. 

'The  dorsal  surface  of  the  thalamus  is  slightly  convex  and  is  divided  by  a  faint 
groove  into  two  parts:  a  lateral  area,  covered  by  the  lamina  affixa  and  forming 
a  part  of  the  floor  of  the  lateral  ventricle;  and  a  larger  medial  area,  which  forms 
the  floor  of  the  transverse  fissure  of  the  cerebrum.  The  oblique  groove  separat- 
ing these  two  areas  corresponds  to  the  lateral  border  of  the  fornix  (Figs.  154,  155). 
The  lamina  affixa  is  part  of  the  ependymal  lining  of  the  lateral  ventricle  superim- 


Fornix  t    ,  Transverse  fissure  of  the  cerebrum 


Stratum  zonale 

Chorioid  plexus  of  lateral  ventricle 
Lamina  affixa 


Internal  medullary 
lamina 

Chorioid  plexus  of 
third  ventricle 

Third  ventricle 


Lenticular  nucleus 


Stria  medullaris 

■,  Corpus  callosum 
Lateral  ventricle 


Internal  capsule 


Hypothalamic 
nucleus 


Caudate  nucleus 

Stria  tcrminalis 
and  vena  ter- 
minalis 

_  External  medull- 
ary lamina 

--^Anterior  nucleus 
of  thalamus 

«§»-, ^Lateral  nucleus 
of  thalamus 

-  Medial  nucleus 
of  thalamus 


Red  nucleus' ■ 


Substantia  nigra  •' 


Optic  tract 
Basis  pedunculi 


Fig.  155. — Diagrammatic  frontal  section  through  the  human  thalamus  and  the  structures  which 

immediately  surround  it. 


posed  upon  this  part  of  the  thalamus.  It  is  not  present  in  the  sheep,  where  the 
fornix  is  larger  and  the  entire  dorsal  surface  of  the  thalamus  belongs  to  the  floor 
of  the  transverse  fissure.  These  features  are  well  illustrated  in  Figs.  179  and 
180,  as  is  also  the  position  of  the  transverse  fissure.  This  fissure  intervenes  be- 
tween the  thalamus  and  the  cerebral  hemisphere,  and  contains  a  fold  of  pia 
mater,  known  as  the  tela  chorioidea,  of  the  third  ventricle. 

The  medial  surface  of  the  thalamus  forms  the  lateral  wall  of  the  third  ven- 
tricle (Figs.  158,  159).  It  is  covered  by  the  ependymal  lining  of  that  cavity. 
The  medial  surfaces  of  the  two  thalami  are  closely  approximated,  being  separated 


2i6  I  HE    NERVOUS    SYSTEM 

from  each  other  by  the  cleft-like  space  of  the  third  ventricle,  and  are  united  across 
the  median  plane  by  a  short  bar  of  gray  substance,  the  massa  intermedia. 

The  lateral  surface  is  hidden  from  view.  It  lies  against  the  broad  band  of 
fibers,  known  as  the  internal  capsule,  which  connects  the  cerebral  hemispheres 
with  the  lower  levels  of  the  central  nervous  system.  This  surface  is  best  examined 
in  sections  through  the  entire  cerebrum  (Figs.  155-157).  Many  fibers  stream 
out  of  the  thalamus  through  its  lateral  surface  and  enter  the  internal  capsule, 
through  which  they  reach  the  cerebral  cortex.  To  this  important  stream  of 
fibers  the  name  thalamic  radiation  is  applied. 

The  ventral  surface  of  the  thalamus  is  also  covered  from  view  and  lies  on  the 
hypothalamus,  by  which  it  is  separated  from  the  tegmentum  of  the  mesencepha- 
lon ( lugs.  155,  157).  Many  fibers,  representing  such  ascending  tegmental  paths 
as  the  medial  lemniscus,  spinothalamic  tract,  and  brachium  conjunctivum,  enter 
the  thalamus  through  this  surface. 

Structure  of  the  Thalamus. — The  thalamus  consists  chiefly  of  gray  matter, 
within  which  there  may  be  recognized  a  number  of  nuclear  masses.  Its  dorsal 
surface  is  covered  by  a  thin  layer  of  white  matter,  called  the  stratum  zonale, 
which  in  the  region  of  the  pulvinar  consists  in  large  part  of  fibers  derived  from 
the  optic  tract.  On  the  lateral  surface  of  the  thalamus  next  the  internal  cap- 
sule there  are  many  myelinated  fibers,  which  constitute  the  external  medullary 
lamina  (Figs.  155,  156).  The  medial  surface  is  covered  by  a  layer  of  central 
gray  matter,  continuous  with  that  which  lines  the  cerebral  aqueduct  and  forms 
the  floor  of  the  third  ventricle.  This  central  gray  matter  consists  of  neuroglia 
and  of  scattered  nerve-fibers  and  cells  (the  nucleus  paramedianus  of  Malone, 
1910).  Some  of  these  fibers  are  continued  through  the  gray  matter  that  lines 
the  aqueduct  and  the  floor  of  the  fourth  ventricle,  as  the  dorsal  longitudinal 
bundle  of  Schutz  (Fig.  112).  It  is  probable  that  this  portion  of  the  thalamus 
forms  a  center  for  vasomotor  and  visceral  reflexes,  since  lesions  in  this  region 
are  often  accompanied  by  disturbances  in  the  nervous  control  of  the  blood- 
vessels and  viscera  (Edinger,  1911;  Rogers,  1916).  If  this  be  true,  it  is  probable 
that  the  dorsal  longitudinal  bundle  of  Schutz  serves  to  bring  this  thalamic 
mechanism  for  visceral  adjustments  into  connection  with  the  visceral  efferent 
nuclei  of  the  brain. 

From  the  stratum  zonale,  which  clothes  its  dorsal  surface,  there  penetrates 
into  the  thalamus  a  vertical  plate  of  white  matter,  called  the  internal  medullary 
lamina.  This  subdivides  the  thalamus  into  three  parts:  the  anterior,  medial, 
and  lateral  nuclei.     At  the  rostral  extremity  of  its  dorsal  border  the  internal 


THE    DIENCEPHALON    AND    nil:    OPTIC    NERVE 


217 


medullary  lamina  bifurcates  and  includes  between  its  two  Limbs  the  anterior 
nucleus. 

The  anterior  nucleus  (or  dorsal  nucleus)  of  the  thalamus  is  located  in  the 
dorsal  pari  of  the  rostral  extremity  of  the  thalamus  and  penetrates  like  a  wedge 
between  the  medial  and  lateral  nuclei.  It  protrudes  somewhat  above  the 
genera]  level  of  the  dorsal  surface,  forming  the  anterior  tubercle  of  the  thalamus. 
It  receives  a  large  bundle  of  fibers  from  the  mammillary  body,  the  mamillotha- 
lamic tract  or  bundle  of  Vicq  d'Azyr  (Figs.  156,  204,  205),  and  sends  fibers  to  the 
caudate  nucleus  of  the  corpus  striatum  (Fig.  196). 


Taenia  ttcta  Stria*  La 


Corpui  callosum         Ventrtiulus  literati's 


Strat.  tubffenjymalt 


Fasc.  frontcoccipitaUt 


Nucleus  (nittt \xtus 


Nucleus  anterior  t/ialami 
Nucleus  tat.-ratis  Ihalnmi 


Mass.i  fmtrrmedi 


Corpus  subthalatu. 


Substantia  nigra 


Fig.   156. — Frontal  section  through  the  mammillary  body,  thalamus,  and  adjacent  structures. 

Weigert  method.     (Villiger-Piersol.) 

The  medial  nucleus  of  the  thalamus  is  situated  between  the  central  gray- 
matter  of  the  third  ventricle  and  the  internal  medullary  lamina,  which  separates 
it  from  the  lateral  nucleus  except  in  the  caudal  part,  where  the  line  of  separation 
between  the  two  is  not  distinct.  It  is  said  to  receive  fibers  from  the  olfactory 
centers  and  to  send  fibers  to  the  caudate  nucleus  and  the  subthalamus. 

The  lateral  nucleus  of  the  thalamus  is  by  far  the  largest  of  the  three.  It 
extends  farther  caudad  than  the  medial  nucleus  and  includes  all  of  the  pulvinar. 
Through  the  external  medullary  lamina  and  the  internal  capsule  it  sends  fibers 
to  the  cerebral  cortex  in  the  thalamic  radiation  and  receives  corticothalamic 


2l8 


THE    NERVOUS    SYSTEM 


fibers  in  return.  Especially  in  its  ventral  subdivision  it  receives  all  of  the  as- 
cending sensory  tracts  from  the  tegmentum  of  the  mesencephalon,  as  well  as 
libers  from  the  brachium  conjunctivum  and  red  nucleus.  It  is  much  more  richly 
supplied  throughout  with  myelinated  fibers  than  are  the  other  nuclei  of  the  thala- 
mus. 

The  lateral  nucleus  is  subdivided  into  a  dorsal  portion,  the  lateral  nucleus 
proper,  and  a  ventral  part,  better  known  as  the  ventral  nucleus  of  the  thalamus. 
Within  the  latter  are  two  well-defined  nuclear  masses.     The  more  medial  of 


"•     VtuMculm  lateralis 


Taenia  semicircula. 


Nucleus  caudatus 


Nucleus  anterior  thalami 


'JgW  '^k Nucleus  lateralis  thalami 

%SS?T S*"        Nucleus  medialis  thalami 


Cor/us  gemcultltunt  latere, 


Gyrus  dentatus 


Centrum  mediant 

Luys 

Nucleus  semilunaris 
( I'lechsig) 

Zcna  lateralis  - 

Wernicke 

Nucleus  ruber 

Cornu  Ammont* 


Fig.  157.-Frontal  section  through  the  human  pons,  basis  pedunculi,  thalamus  and  adjacent 
structures.     Weigert  method.      (Villiger-Piersol.) 

the  two  is  known  as  the  nucleus  centralis  (nucleus  globosus  or  centrum  media- 
num)  and  is  surrounded  by  a  well-defined  capsule  of  myelinated  fibers  (Fig.  157). 
Ventrolateral  to  this  is  the  well-defined  nucleus  arcuatus,  which  because  of  its 
shape  is  also  called  the  nucleus  semilunaris.  The  pulvinar  is  a  very  large  mass 
which  forms  the  most  caudal  part  of  the  thalamus  and  is  usually  considered  as 
a  part  of  the  lateral  nucleus. 

Function.— The  medial  and  anterior  thalamic  nuclei  are  closely  associated  in 
function  and  from  a  phylogenetic  point  of  view  represent  the  older  part  of  the 
thalamus.     They  serve  as  centers  for  the  more  primitive  thalamic  correlations 


THE    DIEXCEPHALON    AXD    THE    OPTIC    NERVE  2IO, 

such  as  occur  in  lower  vertebrates  that  lack  the  cerebral  cortex  (Herrick,  1917) 
Both  receive  fibers  from  the  olfactory  centers  and  both  send  fibers  to  the  corpus 
striatum,  but  none  to  the  cerebral  cortex  (Sachs,  1909).  There  is  some  evidence 
of  a  clinical  nature  to  show  that  the  activity  of  these  centers  may  be  accompanied 
by  a  crude  form  of  consciousness  (Head  and  Holmes,  1911;  Head.  1918).  Pa- 
tients  in  whom  the  paths  from  the  thalamus  to  the  cortex  have  been  interrupted 
are  aware  of  many  sensations,  but  cannot  discriminate  among  them.  The 
thalamus  seems  to  be  the  chief  center  for  the  perception  of  pain  and  the  affec- 
tive qualities  of  other  sensations,  and  in  this  respect  it  plays  an  important 
role  in  consciousness  independently  of  the  cerebral  cortex. 

The  more  lateral  group  of  centers,  which  includes  the  lateral  nucleus  of  the 
thalamus,  the  pulvinar,  and  the  geniculate  bodies,  is  of  more  recent  origin  and 
has  been  called  the  neothalamus.  They  serve  as  relay  stations  on  the  somatic 
sensory  paths  to  the  cerebral  cortex.  The  medial  lemniscus  and  spinothalamic 
tracts  terminate  in  the  ventral  subdivision  of  the  lateral  nucleus.  In  the  pul- 
vinar and  lateral  geniculate  body  terminate  fibers  from  the  optic  tracts,  while 
the  lateral  lemniscus  ends  in  the  medial  geniculate  body.  From  these  nuclei 
sensory  fibers  of  the  third  order  run  to  the  cerebral  cortex.  The  lateral  nucleus, 
exclusive  of  the  pulvinar,  is  therefore  a  relay  station  on  the  paths  of  cutaneous 
and  deep  sensibility,  and  it  is  connected  with  the  parietal  and  frontal  cortex 
through  the  thalamic  radiation.  The  pulvinar  and  lateral  geniculate  body  are 
stations  on  the  optic  pathway,  and  the  medial  geniculate  body  on  that  for  hearing. 

The  thalamic  radiation  can  best  be  considered  in  detail  after  we  have  ac- 
quired some  familiarity  with  the  structure  of  the  cerebral  hemisphere  (p.  263). 

The  fiber  tract  connections,  established  by  the  various  nuclear  masses  composing  the 
thalamus,  among  themselves  and  with  other  parts  of  the  brain,  are  not  as  yet  well  known. 
This  is  particularly  true  of  the  descending  tracts.  It  is  known  that  from  the  region  of  the 
thalamus  a  large  bundle,  the  thalamo-olivary  tract,  descends  to  the  inferior  olivary  nucleus. 
Some  authors  also  describe  a  thalamospinal  tract  which  arises  in  the  thalamus  and  is  closely 
associated  with  the  rubrospinal  tract. 

It  is  fairly  well  established  that  each  of  the  ascending  sensory  tracts  of  the  tegmentum 
has  its  own  particular  field  of  distribution  within  the  ventral  nucleus  of  the  thalamus;  and  it 
is,  therefore,  probable  that  there  are  corresponding  functional  differences  in  the  various 
subdivisions  of  this  nucleus.  Beginning  at  the  lateral  side  and  passing  mediahvard.  the 
terminals  of  these  various  tracts  are  as  follows:  The  spinothalamic  tract  ends  in  the  most 
lateral  part  of  the  ventral  nucleus.  Next  comes  the  field,  within  which  terminate  the  fibers 
of  the  central  tract  of  the  trigeminal  nerve,  and  which  includes  the  nucleus  arcuatus  and 
nucleus  centralis.  The  medial  lemniscus  ends  in  the  most  medial  part  of  the  inferior  nucleus, 
including  the  nucleus  centralis.  This  corresponds  to  the  relative  position  which  these  tracts 
occupy  in  the  tegmentum  of  the  mesencephalon,  where  the  spinothalamic  tract  is  the  most 
lateral  of  the  three. 


22Q  THE    NERVOUS   SYSTEM 

THE  METATHALAMUS 
The  mctathalamus  is  composed  of  two  small  protuberances,  the  geniculate 
bodies,  which,  having  been  displaced  by  the  excessive  development  of  the 
thalamus,  are  situated  upon  the  dorsolateral  surface  of  the  rostral  end  of  the 
mesencephalon  (Figs.  87-89,  154,  161).  The  lateral  geniculate  body  is  an  oval 
swelling  in  the  course  of  the  optic  tract.  Its  connections  will  be  more  fully 
considered  in  connection  with  the  discussion  of  the  course  of  the  visual  impulses. 
The  medial  geniculate  body  is  overhung  by  the  pulvinar,  from  which  it  is  separated 
by  a  deep  sulcus.  It  receives  fibers  by  way  of  the  inferior  quadrigeminal  bra- 
chium  from  the  lateral  lemniscus,  which  we  have  learned  to  know  as  the  central 
auditory  path  from  the  cochlear  nuclei.  From  it  fibers  run  to  the  auditory 
ana  of  the  cerebral  cortex  (the  thalamotemporal  or  acoustic  radiation). 

THE  EPITHALAMUS 
The  epithalamus  includes  the  pineal  body,  stria  medullaris,  and  habemilar 
trigone.  The  latter  is  a  small  triangular  depressed  area  located  on  the  dorso- 
medial  aspect  of  the  thalamus  rostral  to  the  pineal  body  (Fig.  158) .  In  the  sheep, 
as  in  most  other  mammals,  it  is  much  larger  than  in  man  and  bulges  both  dor- 
sally  and  medially  beyond  the  surface  of  the  thalamus  (Figs.  91,  159).  It  marks 
the  position  of  a  nuclear  mass,  called  the  habenular  ganglion,  which  receives  fibers 
from  the  stria  medullaris,  a  fascicle  which  runs  along  the  border  between  the 
dorsal  and  medial  surfaces  of  the  thalamus  subjacent  to  the  taenia  thalami 
(Figs.  154,  155).  The  stria  medullaris  takes  origin  from  the  anterior  perforated 
substance  and  other  olfactory  centers  on  the  basal  surface  of  the  cerebral  hemi- 
sphere and,  partially  encircling  the  thalamus,  reaches  the  habenular  ganglion, 
in  which  it  ends.  (See  p.  281.)  Not  all  of  the  fibers  terminate  on  the  same 
side;  some  cross  to  the  ganglion  of  the  opposite  side,  forming  a  transverse  bundle 
of  myelinated  fibers  which  joins  the  caudal  end  of  the  two  ganglia  together  and 
is  known  as  the  habenular  commissure.  From  the  cells  in  this  ganglion  arises 
a  bundle  of  fibers,  known  as  the  fasciculus  retroflexus  of  Meynert  or  the  tractus 
habenulopeduncularis.  This  bundle  of  fibers  is  directed  ventralward  and  at 
the  same  time  caudally  along  the  medial  side  of  the  red  nucleus  toward  the 
base  of  the  brain,  where  it  crosses  to  the  opposite  side  and  ends  in  the  inter- 
peduncular ganglion  (Fig.  189).  The  stria  medullaris,  habenular  ganglion, 
and  fasciculus  retroflexus  are  all  parts  of  an  arc  for  olfactory  reflexes,  as  indi- 
cated in  Fig.  211.  According  to  Edinger  (1911)  the  cells,  from  which  the  stria 
medullaris  arises,  are  intimately  related  to  a  bundle  of  ascending  fibers  from 


rill:    D1EXCEPHALON    AND    THE    OPTIC    NERVE 


221 


the  sensory  nuclei  of  the  trigeminal  nerve.  I!"  this  he  true,  the  mechanism  in 
question  may  receive  afferent  impulses  from  the  nose,  mouth,  and  tongue  and 
he  concerned  with  feeding  reflexes. 

The  pineal  body  is  a  small  mass,  shaped  like  a  fir  cone,  which  re>ts  upon  the 
mesencephalon  in  the  interval  between  the  two  thalami.  Its  base  is  attached 
by  a  short  stalk  to  the  habenular  and  posterior  commissures,  and  into  the  stalk 
there  extends  the  small  pineal  recess  of  the  third  ventricle.  The  pineal  body  i- 
a  rudimentary  structure  and  is  not  composed  of  nervous  elements.     In  some 


Posterior  com  m  issun 
Pirn  ill  body 
Splcnium  of  corpus  callosum      - 
Lamina  quadrigemina  x   \ 
Ccnhrai  aqueduct  s   \ 
Anterior  medullary  velum  s     \ 

Four Ui  ventricle  ^ 
Sup.  verm,  of  cerebellum  „ 
Fissura  prima   . 


£      L 

>: 

Inferior  vermis  ,,-        M 
of  cerebellum 


Hypothermic  sulcus        ,  Body  of  fornix 

II abeu  ula  ^       \      ,  Chorioid 'plexus  of  third  ventricle 

Habenular  commissure       l      i     i        /       Massa  intermedia 
Suprapineal  recess       \     •     '    '      /    /      Epithelial  roof  of  third  ventricle 

I     i  Lamina  commissures  hippocampi 
(  Corpus  col  I  os  urn 


Epithelial  roof  and  chori- 
oid plexus  of  fourth-'' 
ventricle 


Genu  of  corpus 

callosum 

/-^„       Septum  pclluci- 

dum 
^-~  Ros.ofcor.  callosum 
Lamina  rostral  is 
„  "'  Columna  fornicis 
-  "•  Interventricular  foramen 
«^v  Anterior  commissure 
N,/ Lamina  terminalis 
* -COptic  recess 
'^^  Optic  chiasma 
^^Infundibulum 
\    \       \  \        Hypophysis 
»    \  \   Mammillary  body 

*  *  Oculomotor  nerve 

\     \  ySi<bthalamus 
\   Tegmentum  of  mesencephalon 
Pons 
-  Medulla 


'* -Central  canal 
Fig.  158. — Median  sagittal  section  through  the  human  brain  stem. 


vertebrates,  certain  lizards  for  example,  it  is  more  highly  developed,  resembles 
in  structure  an  invertebrate  eye,  and  lies  close  to  the  dorsal  surface  of  the  head. 
The  posterior  commissure  is  a  large  bundle  of  fibers  which  crosses  the  median 
plane  dorsal  to  the  point  where  the  cerebral  aqueduct  opens  into  the  third 
ventricle  (Figs.  154,  156).  The  source  and  termination  of  the  fibers  which 
constitute  the  bundle  are  still  obscure. 


222  THE    NERVOUS    SYSTEM 

THE  HYPOTHALAMUS 

The  hypothalamus  consists  of  three  parts:  (1)  the  pars  optica  hypothalami, 
which  belongs  to  the  telencephalon.  (2)  the  pars  mamillaris  hypothalami,  and 
(3)  the  subtJialamus. 

The  pars  mamillaris  hypothalami  includes  the  corpora  mamillaria,  tuber 
cinereum.  infundibulum.  and  hypophysis.  The  mammiUary  bodies  are  a  pair  of 
small  spheric  masses  of  gray  matter,  situated  close  together  in  the  interpedun- 
cular space  rostral  to  the  posterior  perforated  substance  (Figs.  86,  158,  159). 
Each  is  enclosed  in  a  white  capsule  and  projects  as  a  rounded  white  eminence 
at  the  base  of  the  brain  (Fig.  156).  In  the  sheep's  brain  the  two  are  fused  to- 
gether into  a  single  eminence  (Fig.  83).  Each  mammillary  body  is  composed 
of  two  nuclear  masses:  a  large  medial  group  of  small  cells  and  a  smaller  lateral 
collection  of  large  cells.  The  white  capsule  is  formed  by  fibers  from  the  hippo- 
campus, which  sweep  in  a  broad  curve  around  the  thalamus,  forming  a  bundle 
known  as  the  fornix  (Figs.  204,  205).  This  descends  in  front  of  the  interventric- 
ular foramen  and  reaches  the  mammillary  body,  within  which  a  large  part  of 
these  fibers  end.  From  the  dorsal  aspect  of  the  medial  nucleus  springs  a  stout 
fascicle,  which  runs  dorsally,  to  end  in  the  anterior  nucleus  of  the  thalamus,  and 
is  known  as  the  mammillothalamic  tract  or  bundle  of  Yicq  d'Azyr  (Figs.  156.  204, 
205).  A  short  distance  from  the  mammillary  bod}-  there  branches  off  from  this 
tract  another,  the  mammillotcgmcntal  tract  of  Gudden.  which  runs  caudally  in 
the  tegmentum  of  the  mesencephalon  and  probably  ends  in  the  dorsal  tegmental 
ganglion.  The  lateral  nuclear  mass  is  also  connected  with  the  tegmentum  by 
way  of  the  peduncle  of  the  mammillary  body  (Fig.  211). 

The  tuber  cinereum,  as  seen  -from  the  ventral  surface  of  the  brain  (Figs. 
83,  86),  is  a  slightly  elevated  gray  area  rostral  to  the  mammillary  bodies.  It  is 
one  of  the  olfactory  centers.  To  it  there  is  attached  the  funnel-shaped  stalk 
of  the  hypophysis,  known  as  the  infundibulum.  The  hypophysis  is  a  small 
gland  of  internal  secretion,  which  is  not  composed  of  nervous  tissue  and  which 
interests  us  here  only  because  its  posterior  portion  is  developed  as  an  outpock- 
eting  of  the  ventral  wall  of  the  diencephalon,  to  which  it  remains  attached  by 
the  infundibulum.  A  detailed  account  of  this  structure  may  be  found  in  the 
papers  by  Tilney  (1911  and  1913)  listed  in  the  Bibliography  at  the  end  of  this 
volume. 

The  subthalamus  is  situated  between  the  thalamus  and  the  tegmentum  of 
the  mesencephalon  and  forms  a  zone  of  transition  between  these  two  struc- 
tures (Figs.  156,  157).     The  long  sensory  tracts  of  the  tegmentum  run  through 


TIIK    1)1K\(  Ki'HAI.OX    AND    THE    OPTIC    NERVE 


223 


it  on  their  way  to  the  thalamus.     The  red  nucleus  and  the  substantia  nigra 

project  upward  into  it  from  the  mesencephalon.  An  additional  mass  of  gray 
matter  is  found  in  this  region  lateral  to  the  red  nucleus  and  ventral  to  the  thala- 
mus. It  is  known  as  the  hypothalamic  nucleus  and  has  the  shape  of  a  biconvex 
lens.  Its  function  and  fiber  connections  are  not  well  understood;  but  it  is  prob- 
ably a  motor  coordination  center  receiving  fibers  from  the  thalamus,  corpus 
striatum,  and  pyramidal  tract,  and  sending  fibers  downward  in  the  cerebral 

peduncle. 

THE  THIRD  VENTRICLE 

Since  the  third  ventricle  is  chiefly  surrounded  by  structures  belonging  to  the 
diencephalon,  it  will  be  convenient  to  consider  it  at  this  point  and  to  give  at 

Interventricular  foramen  Body  of  corpus  callosum 


Anterior  commissure 
Septum  pellucidumx  \ 

Rostra!  lamina    \ 


Rostrum  of  corpus  callosum  \  \ 
Genu  of  corpus  callosum,  \  \  \ 


Bod'v  of  fornix 

>    Hippocampal  com.  Roofs  of  third  ventricle  or  tela  chorioidca 
\  \   Stria  med.  /Haben.  com. 

\  \  \    Habcnular     /  /Splcnium  Suprapineal  recess 

,  \  \  \  ■      Trigone  ///Pineal        /', Superior  colliculus 
J  /  /  /Primary  fissure 

/White  center  of  vermis 


Olfactory  bulb    . 
Medial  olfactory  gyrus' /  / 
Anterior  perf.  substance'/, 
Lamina  terminalis  / 
Diagonal  band 


W//, 

Tnfundib. 
•  /  ,'      Third  vent. 
!  /  Massa  intermedia 
'    Optic  c  hi  as  ma 
Preoptic  recess 


\  1  'Pons 
^  \  -Aqueduct 

\Lamina  quad. 
\  Posterior  com. 
>. v  Hypophysis 
Mam  miliary  body 


Central  canal 
'  Medulla 
v   1  Medial  aperture  of 
\  \     fourth  ventricle 
\  \Tcla  chorioidca 
\   '  Fourth  ventricle 
x Anterior  medullary 
velum 


Fig.  159. — Medial  sagittal  section  of  the  sheep's  brain. 

the  same  time  an  account  of  the  parts  of  the  telencephalon  which  help  to  form 
its  walls.  These  include  the  lamina  terminalis,  anterior  commissure,  and  the 
optic  chiasma  (Figs.  158,  159).  The  latter,  formed  by  the  decussation  of  the 
fibers  of  the  optic  nerve,  projects  as  a  transverse  ridge  in  the  floor  of  the  ven- 
tricle. The  lamina  terminalis  is  a  thin  plate  joining  the  two  hemispheres,  which 
stretches  from  the  optic  chiasma  in  a  dorsal  direction  to  the  anterior  commis- 
sure. Here  it  becomes  continuous  with  the  thin  edge  of  the  rostrum  of  the 
corpus  callosum,  known  as  the  rostral  lamina.    As  indicated  on  page  26,  the 


224  THE   NERVOUS    SYSTEM 

lamina  terminalis  is  to  be  regarded  as  forming  the  rostral  end  of  the  brain; 
and  the  part  of  the  third  ventricle,  which  lies  behind  it  and  dorsal  to  the  optic 
chiasma,  belongs  to  the  telencephalon.  The  anterior  commissure  is  a  bundle  of 
fibers  which  crosses  the  median  plane  in  the  lamina  terminalis  and  serves  to 
connect  certain  parts  of  the  two  cerebral  hemispheres,  which  are  associated  with 
the  olfactory  nerves.  The  anterior  commissure  and  the  lamina  terminalis  form 
the  rostral  boundary  of  the  third  ventricle,  and  between  the  latter  and  the  optic 
chiasma  is  a  diverticulum,  known  as  the  optic  recess. 

The  third  ventricle  is  a  narrow  vertical  cleft,  the  lateral  walls  of  which  are 
formed  for  the  greater  part  by  the  medial  surfaces  of  the  two  thalami.  Ventral 
to  the  massa  intermedia  is  seen  a  groove  known  as  the  hypothalamic  sulcus,  which 
if  followed  rostrally  leads  to  the  interventricular  foramen,  while  in  the  other 
direction  it  can  be  traced  to  the  cerebral  aqueduct.  Below  this  groove  the 
lateral  wall  and  floor  of  the  ventricle  are  formed  by  the  hypothalamus. 

In  the  floor  of  the  ventricle  there  may  be  enumerated  the  following  structures, 
beginning  at  the  rostral  end:  the  optic  chiasma,  infundibulum,  tuber  cinereum, 
mammillary  bodies,  and  the  subthalamus. 

The  roof  of  the  third  ventricle  is  formed  by  the  thin  layer  of  ependyma,  which 
is  stretched  between  the  striae  medullares  thalami  of  the  two  sides,  and  whose 
torn  edge,  in  the  dissected  specimen,  is  represented  by  the  taenia  thalami  (Figs. 
85,  155,  159).  Upon  the  outer  surface  of  this  ependymal  roof  is  a  fold  of  pia 
mater  in  the  transverse  fissure.  This  is  known  as  the  tela  chorioidea;  and  from 
it  delicate  vascular  folds  are  invaginated  into  the  ventricle,  carrying  a  layer  of 
ependyma  before  them  by  which  they  are,  in  reality,  excluded  from  the  cavity. 
These  folds  are  the  chorioid  plexuses.  There  are  two  of  them  extending  side  by 
side  from  the  interventricular  foramina  to  the  caudal  extremity  of  the  roof. 
Here  they  extend  into  an  evagination  of  the  roof  above  the  pineal  body,  known 
as  the  suprapineal  recess. 

There  are  three  openings  into  the  third  ventricle.  The  aqueduct  of  the  cere- 
brum opens  into  it  at  the  caudal  end;  while  at  the  opposite  extremity  it  com- 
municates with  the  lateral  ventricles  through  the  two  interventricular  foramina. 

THE  VISUAL  APPARATUS 
Development  of  the  Retina  and  Optic  Nerve.— There  is  but  one  pair  of 
nerves  associated  with  the  diencephalon,  and  these,  the  optic  nerves,  are  not 
true  nerves,  but  fiber  tracts  joining  the  retinae  with  the  brain.  It  will  be  re- 
membered that  the  retina  develops  as  an  evagination  of  the  lateral  wall  of  the 
prosencephalon  in  the  form  of  a  vesicle  whose  cavity  is  continuous  with  that  of 


THE    nil  \<  l  I'll  \l.<i\    AM)    THE    OPTIC    NERVE 


225 


the  forebrain.     By  a  folding  of  its  walls  in  the  reverse  direction,  i.  <■.,  by  invag 
ination.  the  optic  vesicle  becomes  transformed  into  the  optic  cup  'M.Lr.  15);  and 
the  cavity  of  the  vesicle  becomes  reduced  to  a  mere  slit  between  the  two  layers 
Forming  the  wall  of  the  cup.    The  inner  of  these  two  layers  develops  into  the 

nervous  portion  of  the  retina;  and  nerve-fibers  arising  in  it  grow  back  to  the  brain 
along  the  course  of  the  optic  stalk,  which  still  connects  the  optic  cup  with  the 
forebrain.  This  mode  of  development  serves  to  explain  why  the  structure  of 
the  retina  resembles  that  of  the  brain  more  than  it  does  that  of  other  sense 
organs,  and  why  the  optic  nerve-fibers,  like  those  of  the  fiber  tracts  of  the  cen- 
tral nervous  system,  are  devoid  of  neurilemma  sheaths.  These  fibers  take  origin 
from  the  ganglion  cells  of  the  retina,  the  structure  of  which  must  be  briefly  con- 
sidered at  this  point. 


Ganglionic  Stratum  opticum 
/nitrons   \Ganglionic  layer 

\  Inner  molecular  layer 

Bipolar    J      inner  nuclear  layer 
neurons    I 

!  Outer  molecular  layer 

_   ,        ,1       Outer  nuclear  layer 
Rod  and 

cone      ;  Ex.  limiting  membrane 
neurons    |  Layer  of  rods  and  cones 


Optic  nerve 

Optic  chiasma 

! 


Optic  tract 

V 

Lateral  geniculate  body 

Medial  geniculate  body 
j\~"  Pulvinar 

Superior  colliculus 


Fig.  160. — Schematic  representation  of  the  retina  and  the  connections  established  by  the  optic 

nerve-fibers. 

The  retina  presents  for  consideration  three  layers  of  superimposed  nervous 
elements:  (1)  the  visual  cells,  (2)  the  bipolar  cells,  and  (3)  the  ganglion  cells 
(Fig.  160).  These,  with  some  horizontally  arranged  association  neurons  and 
supporting  elements,  form  the  nervous  portion  of  the  retina  and  are  derived 
from  the  inner  layer  of  the  optic  cup.  The  pigmented  stratum  of  the  retina  is 
derived  from  the  outer  layer  of  the  cup. 

The  visual  cells  are  bipolar  elements,  whose  perikarya  are  located  in  the 
outer  nuclear  layer  (Fig.  160).  Each  presents  an  external  process  in  the  form  of 
a  rod  or  cone,  so  differentiated  as  to  respond  to  photic  stimulation  and  thus  to 
serve  as  a  visual  receptor.  The  other  process  terminates  in  the  outer  molecular 
layer  in  relation  to  processes  from  the  bipolar  cells.  These  latter  elements  have 
their  perikarya  in  the  inner  nuclear  layer  and  branches  in  the  inner  and  outer 
molecular  layers.  The  ganglion  cells  send  their  dendrites  into  the  inner  molec- 
ular layer,  where  they  are  related  to  the  inner  branches  of  the  bipolar  cells; 
15 


226 


THE    NERVOUS    SYSTEM 


while  the  axons  form  the  innermost  stratum  of  the  retina,  the  stratum  optician, 
through  which  they  enter  the  optic  nerve.  It  will  be  apparent  from  Fig.  160 
that  the  visual  cells  are  the  receptors  and  neurons  of  the  first  order  in  the  optic 
path.  The  impulses  are  transmitted  through  the  bipolar  cells  to  the  ganglion 
cells,  whose  axons,  in  turn,  carry  them  by  way  of  the  optic  nerves  to  the  supe- 
rior colliculus.  lateral  geniculate  body,  and  pulvinar  of  the  thalmus.  In  the  same 
figure  it  may  be  seen  that  the  nerve  also  contains  some  efferent  fibers  which 
terminate  in  the  retina  ( Arey.  1916). 

The  Optic  Chiasma  and  Optic  Tracts. — The  optic  nerve  emerges  from  the 
bulbus  oculi  at  the  nasal  side  of  the  posterior  pole  and.  after  entering  the  cranium 
through  the  optic  foramen,  unites  with  its  fellow  of  the  opposite  side  to  form  the 

Pulvinar  of  thalamus     Aqueduct  of  cerebrum     Red  nucleus 
Medial  geniculate  body.  -      —  ■-'      y   /Tegmentum 


Lateral  geniculate  body 


Cerebral  peduncle 
Optic  tract- 

Posterior  perforated  substance '' 


Substantia  nigra 

Base  of  peduncle 


Mammillary  body 


or  perforated  substance 


Tuber  cinereum 

Optic  nerve 


Optic  chiasma 


Olfactory  trigone 
ndibulum 


Fig.  161. — The  connections  and  relations  of  the  optic  tracts.     The  mesencephalon  has  been  cut 
across  and  the  specimen  is  viewed  from  below.     (Sobotta-McMurrich  . 

optic  chiasma.  in  which  a  partial  decussation  of  the  fibers  takes  place  (Tigs. 
161.  162).  Beyond  the  decussation  fibers  from  both  retinae  are  continued  in 
each  of  the  optic  tracts.  In  the  chiasma  the  fibers  from  the  two  optic  nerves 
are  so  distributed  that  each  tract  receives  the  fibers  from  the  lateral  half  of  the 
retina  of  its  own  side  and  those  from  the  medial  half  of  the  opposite  retina. 
The  optic  tracts  partially  encircle  the  ends  of  the  cerebral  peduncles.  Each 
tract  divides  into  a  medial  and  a  lateral  root,  of  which  the  former  goes  to  the 
medial  geniculate  body  and  does  not  consist  of  optic  nerve-fibers.  The  lateral 
root  is  much  larger  and  runs  to  the  lateral  geniculate  body  and  pulvinar  of  the 
thalamus  and  to  the  superior  colliculus  of  the  corpora  quadrigemina.  In  addi- 
tion to  the  optic  fibers  each  tract  contains  a  bundle  of  fibers,  known  as  the  com- 


III!.     DIK\<   I   I'HAI.ON     AM)     I  III.     "M  [I       M   R\  I 


2  2~, 


missure  of  Gudden,  which  crosses  the  median  plane  in  the  posterior  part  of  the 
optic  chiasma  and.  for  the  most  part  at  least,  connects  the  medial  geniculate 
bodies  of  the  two  sides.  These  are  the  libers  which  form  the  medial  root  of  the 
optic  tra<  I . 

The  Optic  Radiation. — The  superior  colliculus  is  a  reflex  center,  and  the  fibers 
of  the  optic  nerve,  which  terminate  in  it,  subserve  optic  reflexes.  On  the  other 
hand,  the  visual  impulses,  brought  to  the  external  geniculate  body  and  the  pul- 

~~  Superior  oblique  mu 
—Retina 

/?/  _^-r^--  — 'Optic  nerve 

-  Optic  chiasma 

"Commissure  of  Gudden 
"  Trochlear  nerve 
■-O.ptie  tract 
—Thalamus 

"Medial  geniculate  body 
'--Lateral  geniculate  body 
•"■Pulvinar 
-Superior  colliculus 
"'Inferior  colliculus 
"^Nucleus  of  trochlear  nerve 
-^Optic  radiation 

--C uncus 


Occipital  pole 

Fig.  162. — Schematic  representation  of  the  optic  pathways.     The  index  line  to  the  commissure  of 

Gudden  does  not  reach  that  structure. 


vinar  of  the  thalamus,  are  relayed  to  the  cerebral  cortex  and  give  rise  to  visual 
sensations.  These  two  parts  of  the  diencephalon  are  connected  with  the  cere- 
bral cortex  on  both  sides  of  the  calcarine  fissure  by  projection  fibers,  which 
form  a  conspicuous  bundle  that  sweeps  backward  through  the  retrolenticular 
portion  of  the  internal  capsule  into  the  occipital  lobe.  It  is  known  as  the  optic 
radiation  (Fig.  162).  In  addition  to  cortici petal  fibers  arising  in  the  pulvinar 
and   lateral   geniculate   body,    the    optic   radiation   contains   corticifugal  fibers 


221 


THI.    NERVOUS    SYSTEM 


arising  in  the  cortex  and  terminating  in  the  pulvinar.  lateral  geniculate  body, 
and  superior  colliculus  of  the  corpora  quadrigemina. 

The  significance  of  the  partial  decussation  of  the  nerves  is  made  clear  by 
Figs.  162  and  163.  The  properties  of  the  refracting  media  of  the  eyes  are  such 
that  images  of  objects  to  the  left  of  the  axis  of  vision  are  produced  on  the  nasal 
side  of  the  left  eye  and  the  temporal  side  of  the  right  eye.  And.  due  to  the  man- 
ner of  decussation  of  the  optic  nerve-fibers,  impulses  from  both  these  sources 
reach  the  visual  area  of  the  right  cortex.  In  the  same  way  the  visual  cortex 
of  the  left  side  receives  impressions  from  objects  to  the  right  of  the  axis  of  vision. 
That  is  to  say.  the  sensory  representation  of  the  outer  world  in  the  cerebral 
cortex  is  contralateral  in  the  case  of  sight  just  as  it  is  in  the  case  of  cutaneous 


Fig.  163. — Diagram  to  show  why  a  destruction  of  one  optic  tract  causes  blindness  in  both  eyes  for 
the  opposite  lateral  half  of  the  field  of  vision. 


and  auditory  sensations.  Furthermore,  it  will  be  evident  that,  while  destruc- 
tion of  one  optic  nerve  causes  total  blindness  in  the  corresponding  eye.  destruc- 
tion of  one  optic  tract,  its  thalamic  connections,  their  optic  radiations,  or  the 
visual  cortex  in  which  these  radiations  terminate,  will  produce  blindness  in  both 
eyes  for  the  opposite  lateral  half  of  the  field  of  vision.  This  condition  is  known 
as  hemianopsia,  and  is  produced  by  a  lesion  in  the  optic  pathway  anywhere  be- 
hind the  chiasma. 


CHAPTER  XV 

THE  EXTERNAL  CONFIGURATION  OF  THE  CEREBRAL 
HEMISPHERES 

Development. — The  cerebral  hemispheres  are  formed  by  the  examination  of 
the  alar  lamina'  of  the  telencephalon,  tin-  rest  of  which  remains  a-  tin-  boundary 
of  the  rostral  part  of  the  third  ventricle,  and  i-  known  as  the  telencephalon 
medium.  The  cavities  of  the  evaginated  portions  are  known  as  the  lateral  ven- 
tricles and  communicate  with  the  third  ventricle  by  way  of  the  interventricu- 
lar foramina  (Figs.  15  17).  Each  of  the  cerebral  hemispheres  consists  of  two 
ventrally  placed  portion-,  the  rhinenceplialon  or  olfactory  lobe  and  corpus  stria- 
tum, and  a  third  part,  more  extensive  than  the  others,  the  pallium  or  primitive 
cerebral  cortex.  The  pallium  expands  more  rapidly  than  the  other  parts,  both 
rost rally  and  caudally.  and  comes  to  overlie  the  diencephalon.  from  which  it  is 
separated  by  the  transverse  fissure  (Fig.  17).  The  fold  of  pia  mater  which  is 
inclosed  within  this  fissure  is  known  as  the  tela  ehorioidea;  and  from  it  a  vascular 
plexus  grows  into  the  lateral  ventricle  through  the  thin  portion  of  the  medial 
wall  of  the  hemisphere,  where  this  is  attached  to  the  diencephalon.  This  forms 
the  chorioid  plexus  of  the  lateral  ventricle  and  carries  before  it  an  epithelial  cover- 
ing from  the  ependymal  lining,  by  which  it  is.  in  reality,  excluded  from  the 
ventricular  cavity.  This  invagination  of  the  medial  wall  of  the  hemisphere 
produces  the  chorioid  fissure.  Ventrally  the  thickened  part  of  the  hemisphere, 
known  as  the  corpus  striatum,  remains  in  uninterrupted  continuity  with  the 
thalamus. 

At  first  the  cerebral  hemisphere  has  a  relatively  large  cavity  and  thin  walls. 

As  the  pallium  and  ventricle  enlarge  they  become  bent  around  the  thalamus 

and  corpus  striatum   (Fig.    17).     The  hemisphere  becomes  bean  shaped  and 

the  cavity  curved.     It  expands  rostrally  to  form  the  frontal  lobe,  caudally  to 

form  the  occipital  lobe,  and  ventrolaterally  to  form  the  temporal  lobe  (Fig.  164). 

Into  each  of  these  there  is  carried  a  prolongation  of  the  lateral  ventricle  forming 

respectively  the  anterior,  posterior,  and  inferior  horns.     Between  the  temporal 

and  frontal  lobes  a  deep  fossa  appears  which  is  the  forerunner  of  the  lateral 

fissure.     At  the  bottom  of  this  fossa  is  the  insula,  a  portion  of  the  cortex  which 

229 


230 


THE    NERVOUS    SYSTEM 


overlies  the  corpus  striatum  and  develops  more  slowly  than  the  surrounding  areas 
(labelled  lateral  fissure.  Fig.  164).  Folds  from  the  surrounding  cortex  close  in 
over  the  insula,  burying  it  from  sight  in  the  adult  brain.  These  folds  are  known 
as  the  opcrciila.  and  the  deep  cleft  which  separates  them  as  the  lateral  fissure. 

Development  of  the  Cerebral  Cortex. — At  first  the  pallium,  like  other  parts 
of  the  neural  tube,  consists  of  three  primitive  zones:  the  ependymal,  mantle, 
and  marginal  layers.  But  during  the  third  month  neuroblasts  migrate  outward 
from  the  ependymal  and  mantle  layers  into  the  marginal  zone  and  there  give 
rise  to  a  superficial  layer  of  gray  matter — the  cerebral  cortex.  Nerve-fibers 
from  these  neuroblasts  and  others  growing  into  the  hemisphere  from  the  thala- 


Sidcus  postccntralis 


Sulcus  centralis 


Lohus 
parietalis 

superior 

Supra- 
marginal 
and  an- 
gular gyri 
Post. 
ramus 
of  lateral 
fissure 


Middle 

temporal 

sulcus 

Occipital 
pole 


Inferior 

frontal 

sulcus 


Temporal 
lobe 


Superior  temporal  gyrus         Middle  temporal  gyrus 
Fig.  164. — Lateral  view  of  the  right  cerebral  hemisphere  from  a  seven  months'  fetus.    (Kollmann.) 

mus  accumulate  on  the  deep  surface  of  the  developing  cortex  and  form  the 
white  medullary  substance  of  the  hemisphere.  As  the  brain  increases  in  size 
the  area  of  the  cortex  expands  out  of  proportion  to  the  increase  in  volume  of 
the  white  medullary  layer  upon  which  it  rests,  and  is  thrown  into  folds  or  gyri 
separated  by  fissures  or  sulci.  All  the  larger  mammalian  brains  present  well- 
developed  gyri,  while  the  smaller  brains  are  smooth;  and  it  would  thus  appear 
that  the  size  of  the  brain  is  an  important  factor  in  determining  the  amount  of 
folding  that  occurs  in  the  cortex. 

As  we  shall  learn,  the  cortex  does  not  differentiate  in  exactly  the  same  man- 
ner throughout,  but  may  be  subdivided  into  structurally  and  functionally  dis- 


THE  EXTERNAL  CONFIGURATION  OF  THE  CEREBRAL  HEMISPHERES    231 

tinct  areas.  The  sulci  develop  in  more  or  less  definite  relation  to  these  areas, 
the  great  majority  making  their  appearance  along  the  boundary  lines  between 
them.  These  are  known  as  terminal  sulci,  of  which  the  rhinal  fissure  and  central 
sulcus  are  examples.  Sometimes  the  folding  occurs  entirely  within  such  an 
area,  i.  <\,  along  its  axis,  producing  what  is  known  as  an  axial  sulcus.  But 
there  are  still  others  in  which  the  relation  to  these  functional  areas  is  not  so  evi- 
dent. The  arrangement  of  the  fissures  and  sulci  in  a  seven  month  fetus  is  shown 
in  Fig.  164. 

The  Development  of  the  Septum  and  Commissures. — The  two  hemispheres 
are  connected  by  the  lamina  terminalis,  which  serves  as  a  bridge  for  fibers  which 
cross  from  one  hemisphere  to  the  other.     These  form  three  important  bundles: 


Fig.  165. — Schematic  representation  of  the  development  of  the  septum  pellucidum  and 
telencephalic  commissures:  A.  C,  Anterior  commissure;  C.  C,  corpus  callosum;  C.  F.,  columna 
fornicus;  C.  S.  P.,  cavum  septi  pellucidi;  F.,  fornix;  H.  C,  hippocampal  commissure;  /.  F., 
interventricular  foramen;  Fis.,  chorioid  fissure;  L.  T.,  lamina  terminalis.  (Based  on  drawings  of 
models  of  the  telencephalon  of  a  four  months'  fetus  (.4)  and  of  a  five  months'  fetus  (B)  by  Streeter.) 

the  anterior  commissure,  the  hippocampal  commissure,  and  the  corpus  callosum. 
The  two  former  connect  the  olfactory,  portions  of  the  hemispheres,  while  the 
latter  is  the  great  commissure  of  the  non-olfactory  cortex  or  neopallium.  Every- 
one admits  that  the  anterior  commissure  develops  in  the  lamina  terminalis 
(Fig.  165);  and  the  corpus  callosum  and  hippocampal  commissures  are  said  to 
form  in  its  dorsal  part  (Streeter,  1912).  According  to  this  account  the  lamina 
terminalis  becomes  stretched  by  the  great  development  of  the  corpus  callosum 
and  appropriates  part  of  the  paraterminal  body.  This  is  the  portion  of  the 
rhinencephalon  that  lies  immediately  rostral  to  the  lamina  terminalis  in  the 
medial  wall  of  each  hemisphere.  Eventually  the  lamina  terminalis  presents  a 
large  cut  surface  in  the  median  sagittal  section  and  includes  the  commissures 


232 


THE    NERVOUS    SYSTEM 


as  well  as  the  septum  pellucidum.  The  portion  of  the  lamina  terminalis  which 
enters  into  the  formation  of  the  septum  becomes  hollow  as  a  result  of  the  stretch- 
ing to  which  it  is  subjected,  and  the  resulting  cavity  is  known  as  the  cavum  septi 
pellucidi. 

The  cerebral  hemispheres  are  incompletely  separated  from  each  other  by 
the  longitudinal  fissure  of  the  cerebrum,  at  the  bottom  of  which  lies  a  broad  band 
of  commissural  fibers,  the  corpus  callosum,  which  forms  the  chief  bond  of  union 
between  them.  Each  hemisphere  has  three  surfaces:  a  convex  dorsolateral 
surface  (Fig.  166),  a  median  surface  flattened  against  the  opposite  hemisphere 
(Fig.  170),  and  a  very  irregular  ventral  or  basal  surface.  A  dorsal  border  sepa- 
rates the  dorsolateral  from  the  medial  surface;  and  a  lateral  border  marks  the 
transition  between  the  dorsolateral  and  basal  surfaces.  One  may  recognize 
also  frontal,  occipital,  and  temporal  poles  (Fig.  166).  The  long  axis  of  the  hemi- 
sphere extends  between  the  frontal  and  occipital  poles,  and  in  man  is  placed 
almost  at  right  angles  to  the  long  axis  of  the  body  (Fig.  33) ;  while  in  other  mam- 
mals it  corresponds  more  nearly  to  the  body  axis.  On  this  account  it  will  be 
convenient  in  the  description  of  the  human  cerebral  hemisphere  to  take  the 
occiput  as  a  point  of  reference  and  use  the  term  "posterior"  in  place  of  "caudal." 
Otherwise  our  directive  terms  remain  the  same — rostral,  dorsal,  and  ventral — 
except  that  for  the  term  "ventral"  we  shall  often  use  the  word  "basal." 

The  cerebral  cortex  is  a  layer  of  gray  matter  spread  over  the  surface  of  the 
hemisphere;  and  its  area  is  greatly  increased  by  the  occurrence  of  folds  or  gyri 
separated  by  deep  sulci.  That  part  of  the  cortex  which  belongs  to  the  rhinen- 
cephalon  and  is  phylogenetically  the  oldest  is  designated  as  the  archipallium. 
It  is  separated  from  the  newer  and  in  mammals  much  larger  neopallium  or  non- 
olfactory  cortex  by  the  rhinal  fissure  (Figs.  83,  171). 

The  Neopallium. — The  development  of  the  neopallium  is  so  much  greater 
in  man  than  in  the  sheep,  and  the  arrangement  of  the  gyri  and  sulci  is  so  dif- 
ferent in  the  two  forms  that  but  little  can  be  learned  by  a  cursorv  comparison  of 
these  structures  in  the  two  brains.  We  shall,  accordingly,  confine  our  atten- 
tion almost  exclusively  to  the  arrangement  of  the  neopallium  in  man. 

THE  DORSOLATERAL  SURFACE  OF  THE  HEMISPHERE 

By  means  of  some  of  the  more  important  sulci  the  cortex  is  marked  off  into 
well-defined  areas,  known  as  the  frontal,  parietal,  temporal,  and  occipital  lobes 
(Fig.  167).  To  these  should  be  added  a  lobe  buried  at  the  bottom  of  the  lateral 
fissure  and  known  as  the  insula  (Fig.  169).     In  the  delimitation  of  these  lobes 


THE    EXTERNAL    CONFIGURATION    OP     IIII.    CEREBRAL    HEMISPHE] 


235 


the  lateral  fissure  and  the  central  sulcus  play  a  prominent  part.     Some  of  the 
more  important  sulci  arc  designated  as  fissures.      I  his  usage  is  regulated  by 
custom,  but  it  may  be  said  that  a  number  of  the  assures  are  invaginatioi 
the  entire  thickness  of  the  wall  of  the  hemisphere  and  produce  corresponding 
elevation-  projecting  into  the  lateral  ventricle. 

The  lateral  cerebral  fissure,  or  fissure  of  Sylvius,  begins  on  the  basal  sur- 
face of  the  brain  as  a  deep  cleft  lateral  to  the  anterior  perforated  substance 
(Fig.  172).     From  this  point  it  extends  lateralward  between  the  temporal  and 
frontal  lobes  to  the  lateral  aspect  of  the  brain,  where  it  divides  into  three  branches 
1  iur-.  166,  167).     The  anterior  horizontal  ramus  of  the  lateral  fissure  runs  ros- 


P/ecentral  sulcus 

A  ntcrior  central  gyrus 
Central  sulcus 

"  Posterior  central  gyrus 
Interparietal  sulcus 


Opercular  portion  of  inferior 
frontal  gyrus 


Superior  frontal 
gyrus 


M  iddlc  frontal 
gyrus 


Frontal  pole  -' 
Triangular  portion 
of  inf.  front,  gyrus  (. 

Lateral  cerebral  fissure  \s' 

Temporal  pole' 

Superior  temporal  gyrus 

Superior  temporal  sulcus 

Middle  temporal  gyrus 

Middle  temporal  sulcus 

Inferior  temporal  gyrus 


Supramarginal  gyrus 
L  Interparietal  sulcus 
.  1  ngular  gyrus 

Superior  parietal 

iobule 
Inferior  parietal 

lobule 
Parietooccipital 
fissure 

Lateral  occipital 

gyri 


Occipital  pole 
Transverse  occipital  sulcus 
Superior  temporal  sulcus 
Posterior  limb  of  lateral  cerebral  fissure 


Fig.  166. — Lateral  view  of  the  human  cerebral  hemisphere.      (Sobotta-McMurrich.) 

trally  and  the  anterior  ascending  ramus  dorsally  into  the  frontal  lobe.  The 
posterior  ramus  of  the  lateral  fissure  is  much  longer,  and  runs  obliquely  toward  the 
occiput  and  at  the  same  time  somewhat  dorsally.  The  terminal  part  turns 
dorsally  into  the  parietal  lobe.  This  fissure  is.  in  reality,  a  deep  fossa,  at  the 
bottom  of  which  lies  the  insula.  It  separates  the  frontal  and  parietal  lobes 
which  lie  dorsal  to  it  from  the  temporal  lobe. 

The  central  sulcus  or  fissure  of  Rolando  runs  obliquely  across  the  dorsolateral 
surface  of  the  hemisphere,  separating  the  frontal  from  the  parietal  lobe  (Figs. 
166,  167).  It  begins  on  the  medial  surface  of  the  hemisphere  a  little  behind  the 
middle  of  the  dorsal  border  and  extends  in  a  sinuous  course  rostrallv  and  toward 


234 


I  III     \i  rvm.s   SYSTEM 


the  base,  nearly  reaching  the  posterior  ramus  of  the  lateral  fissure.  It  makes 
an  angle  of  about  70  degrees  with  the  dorsal  border.  It  is  customary  to  recog- 
nize two  knee-like  bends  in  this  sulcus;  one  located  at  the  junction  of  the  dorsal 
and  middle  thirds  with  concavity  forward,  and  the  other  at  the  junction  of  the 
middle  and  basal  thirds  with  concavity  backward.  If  the  margins  of  the  sulcus 
are  pressed  apart  a  deep  annectant  gyrus  may  often  be  seen  extending  across 
it,  by  which  the  continuity  of  the  sulcus  is  to  some  extent  interrupted.  This  is 
explained  by  the  fact  that  the  sulcus  usually  develops  in  two  pieces,  which  be- 
come united  as  the  depth  of  the  sulcus  increases. 

Lobes. — The  frontal  lobe  lies  dorsal  to  the  lateral  cerebral  fissure  and  rostral 
to  the  central  sulcus  (Fig.  167).  The  remainder  of  the  dorsolateral  surface  is 
subdivided  rather  arbitrarily  into  the  parietal,  occipital,   and  temporal  lobes. 

Central  sulcus 


Frontal  lobe 


Frontal  pole  --\ 


—  Parietal  lobe 
s  Temporal  lobe 

.-  Parieto-occipital  fissure 

-  Occipital  lobe 
'pita!  notch 

-Occipital  pole 


Lateral  '  Ant.  hor.  ram.  / 
cerebraliAnt.  ascend .  ram. '  / 
fissun  Post.  ram.  ' 

Temporal  pole'' 

Fig.  167. — Diagram  of  the  lobes  on  the  lateral  aspect  of  the  human  cerebral  hemisphere. 


The  rostral  border  of  the  occipital  lobe  is  usually  placed  at  a  line  joining  the  end 
of  the  parieto-occipital  fissure  with  the  preoccipital  notch.  The  latter  is  a 
slight  indentation  on  the  lateral  border  of  the  hemisphere  about  4  cm.  rostral 
to  the  occipital  pole;  while  the  parieto-occipital  fissure  is  a  deep  cleft  on  the 
median  surface  (Fig.  170),  which  cuts  through  the  dorsal  border  about  midway 
between  the  occipital  pole  and  the  central  sulcus,  but  a  little  nearer  the  former. 
The  parietal  lobe  is  situated  between  the  central  sulcus  and  the  imaginary  line 
joining  the  parieto-occipital  fissure  with  the  preoccipital  notch.  It  lies  dorsal 
to  the  lateral  fissure  and  an  imaginary  line  connecting  that  fissure  with  the 
middle  of  the  preceding  line.  The  remainder  of  the  dorsolateral  surface  belongs 
to  the  temporal  lobe. 

The  Frontal  Lobe. — The  rostral  part  of  the  hemisphere  is  formed  by  the 


THE  EXTERNAL  CONFIGURATION  OF  THE  CEREBRAL  HEMISPHERES 


>35 


frontal  Lobe.  Within  it  one  may  identify  three  chief  sulci,  which  are,  however, 
subject  to  considerable  variation.  The  precentral  sulcus  is  more  or  lessparallel 
with  the  centra]  sulcus  and  is  often  subdivided  into  two  part-,  the  superior  and 
inferior  precentral  sulci  (Fig.  168).  The  superior  frontal  sulcus  usually  begins 
in  the  superior  precentral  sulcus  and  runs  rostrally,  following  in  a  general  way 
the  curvature  of  the  dorsal  border  of  the  hemisphere  which  it  gradually  ap- 
proaches. The  inferior  frontal  sulcus  usually  begins  in  the  inferior  precentral 
sulcus  and  extends  rostrally.  arching  at  the  same  time  toward  the  base  of  the 
hemisphere. 

Between  the  precentral  and  central  sulci  lies  the  anterior  central  gyrus  in 
which  is  found  the  motor  area  of  the  cerebral  cortex.     The  remainder  of  this 


Anterior  central  gyms 

Superior  precentral  sh/chs       ; 

Superior  frontal  gyrus  -- 

Superior  frontal  sulcus -- 
Middle  frontal  gyrus— 
MiddU  frontal  side 
Inferior  frontal  sulcus 
luf  rior  precentral  sulcus  . 
Inf.     Parsopcrcularis 
front.-         Pars  triang.  ■ 
gyrus       Pars  orbitalis- 


Lateral        Ant.  hor.  ram.  ' / 

cerebraHAnt.  ascend,  ram./  y 

fissure  Post,  ram.y'  , 

Superior  temporal  sulcus,'' 

Superior  temporal  gyrus 


,  Central  sulcus 


,  Posterior  central  gyrus 
Postcentral  sulcus 


,  Supra marg.  gyrusl      ';.,' 
^Angular  gyrus        ^^ 

---'  Superior  parietal  lobule 

— -  Interparietal  sulcus 


/  \\  r'  Trails,  occipital  sulcus 
Sulcus  lunatus 


\\  Inferior  temporal  gyrus 
S? Middle  temporal  sulcus 
Middle  temporal  gyrus 

Fig.  168.— Sulci  and  gyri  on  the  lateral  aspect  of  the  human  cerebral  hemisphere. 


surface  of  the  frontal  lobe  is  composed  of  three  convolutions,  the  superior, 
middle,  and  inferior  frontal  gyri.  separated  from  each  other  by  the  superior  and 
inferior  frontal  sulci.  The  inferior  frontal  gyrus,  which  in  the  left  hemisphere 
is  also  known  as  Broca's  convolution,  is  subdivided  by  the  two  anterior  rami  of 
the  lateral  sulcus  into  three  parts,  known  as  the  orbital,  triangular,  and  oper- 
cular portions.  The  orbital  part  of  the  inferior  frontal  gyrus  lies  rostral  to  the 
anterior  horizontal  ramus  of  the  lateral  sulcus;  the  triangular  part  is  a  wedge- 
shaped  convolution  between  the  two  anterior  rami  of  that  fissure;  while  the 
opercular  portion  lies  in  the  frontal  operculum  between  the  precentral  sulcus 
and  the  anterior  ascending  ramus  of  the  lateral  fissure. 

The  Temporal  Lobe.— Ventral  to  the  lateral  fissure  is  the  long  tongue-shaped 


236  THE    NERV01  -    SYS1  EM 

temporal  lobe  which  terminates  rostrally  in  the  temporal  pole.  The  superior 
temporal  sulcus  is  a  very  constant  fissure,  which  begins  near  the  temporal  pole 
and  runs  nearly  parallel  with  lateral  cerebral  fissure.  Its  terminal  part  turns 
dorsally  into  the  parietal  lobe.  The  middle  temporal  sulcus,  ventral  to  the  pre- 
ceding and  in  general  parallel  with  it.  is  usually  composed  of  two  or  more  dis- 
connected parts.  The  interior  temporal  sulcus  is  located  for  the  most  part  on 
the  basal  surface  of  the  temporal  lobe.  Dorsal  to  each  of  these  fissures  is  a 
gyrus  which  bears  a  similar  name:  the  superior  temporal  gyrus,  between  the 
lateral  fissure  and  the  superior  temporal  sulcus;  the  middle  temporal  gyrus,  be- 
tween the  superior  and  middle  temporal  sulci;  and  the  inferior  temporal  gyrus, 
between  the  middle  and  inferior  temporal  sulci.  The  lateral  fissure  is  very  deep; 
and  the  surface  of  the  superior  temporal  gyrus  that  bounds  it  is  broad  and  marked 
near  its  posterior  extremity  by  horizontal  convolutions,  known  as  the  transverse 
temporal  gyri.  One  of  these,  more  marked  than  the  others,  has  been  called  the 
anterior  transverse  temporal  gyrus  or  Heschl's  convolution  and  represents  the 
cortical  center  for  hearing  (Fig.  174). 

The  Parietal  Lobe. — The  postcentral  sulcus  runs  nearly  parallel  with  the 
central  sulcus  and  consists  of  two  parts,  the  superior  and  inferior  postcentral 
sulci,  which  may  unite  with  each  other  or  with  the  interparietal  sulcus.  Often 
all  three  are  continuous,  forming  a  complicated  fissure,  as  shown  in  Fig.  168. 
The  interparietal  sulcus  extends  in  an  arched  course  toward  the  occiput  and 
may  end  in  the  transverse  occipital  sulcus.  These  four  sulci  are  often  included 
under  the  term  ' "interparietal  sulcus."  The  interparietal  sulcus  proper  is  then 
designated  as  the  horizontal  ramus. 

The  posterior  central  gyrus  lies  between  the  central  and  postcentral  sulci. 
The  interparietal  sulcus  separates  the  superior  parietal  lobule  from  the  inferior 
parietal  lobule.  Within  the  latter  we  should  take  note  of  two  convolutions: 
the  supramarginal  gyrus,  which  curves  around  the  upturned  end  of  the  lateral 
fissure;  and  the  angular  gyrus,  similarly  related  to  the  terminal  ascending  por- 
tion of  the  superior  temporal  fissure. 

The  Occipital  Lobe. — Only  a  small  part  of  the  dorsolateral  surface  of  the 
hemisphere  is  formed  by  the  occipital  lobe.  This  is  a  triangular  area  at  the 
occipital  extremity,  bounded  rostrally  by  a  line  joining  the  parieto-occipital 
fissure  and  the  preoccipital  notch  (Fig.  167).  The  transverse  occipital  fissure 
may  help  to  bound  this  area  or  may  lie  within  it.  Other  inconstant  sulci  help 
to  divide  it  into  irregular  convolutions.  Sometimes  the  visual  area  which  lies 
on  the  mesial  aspect  of  this  lobe  is  prolonged  over  the  occipital  pole  to  the  lateral 


THE  EXTERNAL  CONFIGURATION  OE  THE  CEREBRAL  BEMISPHERES 


237 


aspect.  In  this  case  a  small  semilunar  furrow  develops  around  it  on  the  lateral 
surface  and  is  known  as  the  sulcus  lunatus  (Fig.  168).  This  sulcus,  tailed  b) 
Rudinger  the  "Affenspalte,"  forms  a  conspicuous  feature  of  the  lateral  surface  of 
the  cerebral  hemisphere  in  the  lower  ( >ld  World  apes  ( [ngalls,  \{>\  1 

The  Insula.  The  part  of  the  cortex  which  overlies  the  corpus  striatum  la<^> 
behind  in  its  development  and  becomes  overlapped  by  the  surrounding  pallium. 
The  cortex,  which  thus  becomes  hidden  from  view  at  the  bottom  of  the  lateral 
fissure,  forms  in  the  adult  a  somewhat  conical  mass  called  the  insula  or  island  of 
Keil  (Fig.  169).  Its  base  is  surrounded  by  a  limiting  furrow,  the  circular  sulcus, 
which  is,  however,  more  triangular  than  circular,  and  in  which  we  may  recognize 
three  portions:    superior,  inferior,  and  anterior.     The  apex  of  this  conical  lobe 


1'ariclal  lobe 


{ 'entral  sulcus  of  insula 
m^-:  Circular  sulcus 
«#  V       Frontal  lobe 


Occipital  lobe  / , 


^^■■•^  (Pr    Short  gyri  of  insula 

Temporal  lobe  £ong  gyrus  0f  insula 

Fig.  169. — Lateral  view  of  the  human  cerebral  hemisphere  with  the  insula  exposed  by  removal  of 

the  opercula.      (Sobotta-McMurrich.) 

is  known  as  the  limen  insula;  and  the  remainder  is  subdivided  by  an  oblique 
groove  (sulcus  centralis  insula?)  into  the  long  gyrus  of  the  insula  and  a  more 
rostral  portion,  which  is  again  subdivided  into  short  gyri. 

The  Operculum. — As  the  adjacent  portions  of  the  pallium  close  over  the 
insula  (Fig.  164)  they  form  by  the  approximation  of  their  margins  the  three 
rami  of  the  lateral  fissure.  These  folds  constitute  the  opercula  of  the  insula. 
Each  of  the  three  surrounding  lobes  takes  part  in  this  process;  and  we  may 
accordingly  recognize  a  frontal,  a  temporal,  and  a  parietal  operculum  (Fig.  166). 

At  this  point  it  will  be  instructive  to  examine  the  lateral  surface  of  the  cerebral 
hemisphere  of  the  sheep.  It  will  be  seen  that  the  region  which  corresponds  to 
the  insula  is  on  a  level  with  the  general  surface  of  the  hemisphere;  no  opercula 
have  developed,  and  the  lateral  sulcus  is  only  a  shallow  groove  (Fig.  173). 


238 


THE    NERVOUS    SYS  I  EM 


THE  MEDIAN  AND  BASAL  SURFACES 

The  occipital  lobe  comes  more  nearly  being  a  structural  and  functional 
entity  than  any  of  the  other  lobes.  It  corresponds  in  a  general  way  to  the 
"regio  occipitalis"  as  outlined  by  Brodman  (Figs.  216,  217).  and  it  is  probably 
all  concerned  directly  or  indirectly  with  visual  processes.  We  have  seen  that 
it  forms  a  small  convex  area  on  the  lateral  surface  near  the  occipital  pole; 
and  we  now  note  that  it  is  continued  on  to  the  medial  surface  of  the  hemi- 
sphere, where  it  forms  a  somewhat  larger  triangular  field  between  the  parieto- 
occipital and  anterior   portion  of  the   calcarine  fissure  dorsorostrally  and  the 


Sulcus  cinguli 
Sulcus  of  corpus  callosum . 


Body  of  corpus  callosum 

Paracentral  lobule 
Central  sulcus 


Sup.  frontal 
gyrus 

Frontal  por.  of 
sulcus  cinguli 
Frontal  pole 
Genu  of  corp.  cat. 

Septum  pellucidum 
Rosl.  of  corpus  callosum 
A  nlerior  parolfactory  sulcus 

Parolfactory  area    '  ,' 

Temporal  pole     Uncus 
Anterior  commissure       Fimbria 

Hippocampal  gyrus 


^Marginal  portion  of  sulcus  cinguli 
Precuneus 

Column  of  fornix 
ySubparielal  sulcus 
Crus  of  fornix 

.-  Paricto-occip.  fis. 
'Splcn.ofcorp.cal. 
I  "lli.  of  gyrus 
fornicatus 
Cuneus 

Calcarine 
fissure 


-Occipital  poh 


Lingual  gyrus 
Inferior  temporal  gyrus 
Inferior  temporal  sulcus 
Fusiform  gyrus 


Collateral  fissure 
Fasciola  cinerca 


Fig.  170. — Human  cerebral  hemisphere  seen  from  the  medial  side.  The  brain  has  been 
divided  in  the  median  plane  and  part  of  the  thalamus  has  been  removed  along  with  the  mesen- 
cephalon and  rhombencephalon.     (Sobotta-McMurrich.) 

collateral  fissure  ventrally.  On  this  aspect  of  the  brain  it  includes  two  constant 
and  well-defined  convolutions:  the  cuneus  and  the  Ungual  gyrus  (Figs.  170, 
171). 

The  calcarine  fissure  begins  ventrally  to  the  splenium  of  the  corpus  callosum 
and  extends  toward  the  occipital  pole,  arching  at  the  same  time  somewhat 
dorsally.  It  consists  of  two  portions.  The  rostral  part,  the  calcarine  fissure 
proper,  is  deeper,  more  constant  in  form  and  position,  and  phylogenetically 
much  older  than  the  rest,  and  produces  the  elevation  on  the  wall  of  the  lateral 
ventricle  known  as  the  calcar  avis  (Fig.  181).     This  part  terminates  at  the  point 


nil     EXTERNAL   CONFIGURATION    01     nil.    CEREBRAL    HEMISPHERES  239 

when-  the  calcarine  is  joined  by  the  parieto-o<  <  ipital  fissure.  The  other  portion. 
sometimes  called  the  "posterior  calcarine  sulcus,"  arches  downward  and  back- 
ward from  this  junction  toward  the  occipital  pole,  and  occasionally  cuts  across 
the  border  of  the  hemisphere  to  its  dorsolateral  surface.    The  parieto-occipital 

fissure,  which  i>  really  a  deep  fossa  with  much  buried  cortex  at  it.-  depth,  appears 
to  be  the  direct  continuation  of  the  rostral  part  of  the  calcarine  fissure.  It  cuts 
through  the  dorsal  border  of  the  hemisphere  somewhat  nearer  to  the  occipital 

pole  than  to  the  central  sulcus.     These  fissures  form  a  Y-shaped  figure  whose 

stem  is  the  calcarine  figure  and  whose  two  limbs  are  the  parieto-occipital  fissure 
and  the  "posterior  calcarine  sulcus."  If  the  fissures  are  opened  up  the  stem  is 
seen  to  be  marked  off  from  the  two  limbs  by  buried  annectant  gyri. 

The  chucks  is  a  triangular  convolution  with  apex  directed  rostrally.  which 
lie-  between  the  diverging  parieto-occipital  and  calcarine  fissures.  The  rest  of 
the  medial  surface  of  the  occipital  lobe  belongs  to  the  lingual  gyrus,  which  lies 
between  the  calcarine  and  collateral  fissures. 

The  remaining  sulci  and  gyri  on  the  median  and  basal  surfaces  may  now  be 
briefly  described. 

The  sulcus  of  the  corpus  callosum  (sulcus  corporis  callosi)  begins  ventrally  to 
the  rostrum  of  the  corpus  callosum.  encircles  that  great  commissure  on  its  con- 
vex aspect,  and  finally  bends  around  the  splenium  to  become  continuous  with 
the  hi ppocam pal  fissure  (Fig.  171).  The  latter  is  a  shallow  groove,  which  runs 
from  the  region  of  the  splenium  of  the  corpus  callosum  toward  the  temporal 
pole  near  the  dorsomedial  border  of  the  temporal  lobe.  It  terminates  in  the 
bend  between  the  hippocampal  gyrus  and  the  uncus. 

The  sulcus  cinguli  (callosomarginal  fissure)  begins  some  distance  ventral 
to  the  rostrum  of  the  corpus  callosum  and  follows  the  arched  course  of  the 
sulcus  of  the  corpus  callosum.  from  which  it  is  separated  by  the  gyrus  cinguli. 
It  terminates  by  dividing  into  two  branches.  One  of  these,  the  subparietal 
sulcus,  continues  in  the  direction  of  the  sulcus  cinguli  and  ends  a  short  distance 
behind  the  splenium.  The  other,  known  as  the  marginal  ramus,  turns  off  at  a 
right  angie  and  is  directed  toward  the  dorsal  margin  of  the  hemisphere.  A  side 
branch,  directed  dorsally.  is  usually  given  off  from  the  main  sulcus  some  dis- 
tance rostral  to  its  bifurcation,  and  is  known  as  the  paracentral  sulcus. 

The  collateral  fissure  begins  near  the  occipital  pole  and  runs  rostrally.  sepa- 
rated from  the  calcarine  and  hippocampal  fissures  by  the  lingual  and  hippo- 
campal gyri.  It  is  sometimes  continuous  with  the  rliinal  fissure.  The  latter 
separates  the  terminal  part  of  the  hippocampal  gyrus,  which  belongs  to  the  archi- 


2-J.O 


IHI.    NERVOUS    SYSTEM 


pallium,  from  the  rest  of  the  temporal  lobe,  and  is  a  very  conspicuous  fissure  in 
most  mammalian  brains  (Fig.  83). 

Convolutions.— Dorsal  to  the  corpus  callosum  is  the  gyrus  cinguli  between 
the  sulcus  of  the  corpus  callosum  and  the  sulcus  cinguli.  The  superior  frontal 
gyrus  is  continued  over  the  dorsal  border  of  the  hemisphere  from  the  dorso- 
lateral surface  and  reaches  the  sulcus  cinguli.  Surrounding  the  end  of  the 
central  sulcus  is  a  quadrilateral  convolution,  known  as  the  paracentral  lobule. 
It  is  bounded  by  the  sulcus  cinguli,  its  marginal  ramus  and  the  paracentral 
sulcus.  Another  quadrilateral  area,  known  as  the  precuneus,  is  bounded  by 
the  parieto-occipital  fissure,  the  subparietal  sulcus,  and  the  marginal  ramus  of 
the  sulcus  cinguli.     The  hippocampal  gyrus  lies  between  the  hippocampal  fissure 


Superior  frontal  gyrus 

Sulcus  of  corpus  callosum  .v 
Gyrus  cinguli  :, 
Sulcus  cinguli  --. 
Corpus  callosum 
Gyrus  fornicatus 
Frontal  lobe 

Post,  parolfactory  sulcus-^- 
Parolfactory  area- 


S.  centralis 
Paracentral  sulcus 


Ant.  parolfactory  sulcus- 


Paracentral  lobule 

„  ,  •  Parietal  lobe 
_..-   Marginal  ramus 
__.—  Precuneus 

'Subparietal  sulcus 
--  Parieto-occipital  fissure 

—  Cuneus 
--  Cakarine  fissure 

-  Lingual  gyrus 
.    Isthmus  of  gyrus 


Temporal  lobe ' 

Rhinal  fissure' 

Uncm  !  Hippocampal  gyrus      \ 

Inf.  temporal  gyrus 


fornicatus 
~-  Hippocampal  fissure 

Collateral  fissure 
v>  Fusiform  gyrus 

Inferior  temporal  sulcus 


Fig.  171. — Diagram  of  the  lobes,  sulci,  and  gyri  on  the  medial  aspect  of  the  human  cerebral 

hemisphere. 

dorsally  and  the  collateral  and  rhinal  fissures  ventrally.  Its  rostral  extremity 
bends  around  the  hippocampal  fissure  to  form  the  uncus.  It  is  connected  with 
the  gyrus  cinguli  by  a  narrow  convolution,  the  isthmus  of  the  gyrus  fornicatus. 
Under  the  name  gyrus  fornicatus  it  has  been  customary  to  include  the  gyru- 
cinguli.  isthmus,  hippocampal  gyrus,  and  uncus.  Between  the  collateral  fissure 
and  the  inferior  temporal  sulcus  is  the  fusiform  gyrus  which  lies  on  the  basal 
surface  of  the  temporal  lobe  in  contact  with  the  tentorium  of  the  cerebellum 
(Figs.  170.  172 ). 

It  has  been  customary  to  apportion  parts  of  the  medial  and  basal  surfaces 
of  the  cerebral  hemisphere  to  the  frontal,  parietal,  occipital,  and  temporal 
lobes,  as  indicated  in  Fig.  171.     According  to  this  scheme  the  gyrus  fornicatus 


1111      EXTERNAL  CONFIGURATION    OP     I  III     CEREBRAL    II  I.  MI -I'll  I 


241 


stand-  by  it-di'  ami  i-  sometimes  designated  a-  tin-  limbic  lobe.  This  plan  of 
subdivision,  which  was  based  <>n  the  erroneous  belief  that  all  portions  of  the 
gyrus  fornicatus  belonged  t<>  the  rhinencephalon,  should  In-  abandoned.  A 
simpler  ami  more  logical  arrangement  assigns  the  hippocampal  gyrus  and  uncus 
to  the  temporal  lobe  and  divides  the  gyrus  cinguli  between  the  frontal  and 
parietal  lobes. 


Optic  chiasma 


Orbital  gyri 
Anterior  perforated  substance  .. 

/ .  mporal  pole 


Lateral  cerebral 
(Sylvian)  fissure 


Longitudinal  fi  \urt  of  cerebrum 

frontal  pole 

Gyrus  rectus 

^^0//./(  lory  sulcus 

"M    Vj.    Orbital  sulci 

&/  V^k  Olfn  lory  trigone 

Mammillary  body 
.- 1  'ncus 


Middle  temporal  sulcus- 


Tuber  cinereum 


Hippocampal  fissure-  T 

Collateral  fissure-'' 
Inferior  temporal  sulcus 


Cerebral  aqueduct  •' 

Collateral  fissure 


Cuneus 


Middle  temporal  sulcus 
Base  of  cerebral  peduncle 
Substantia  nigra 


'Inferior  temporal  gyrus 


Fusiform  gyrus 

Hippocampal  gyrus 
Corpus  quadrigeminum 
Isthmus  of  gyrus  fornicatus 
Lingual  gyrus 
Gyrus  cinguli 
Splenium  of  corpus  callosum 
Parieto-occipital  fissure 
Occipital  pole 

(Sobotta-McMurrich.) 


Fig.  172. — Basal  aspect  of  the  human  cerebral  hemisphere 

The  basal  surface  of  the  hemisphere  (Fig.  172)  consists  of  two  parts:  (1) 
the  ventral  surface  of  the  temporal  lobe,  whose  sulci  and  gyri  have  been  de- 
scribed in  a  preceding  paragraph,  and  which  rests  upon  the  tentorium  cerebelli 
and  the  floor  of  the  middle  cranial  fossa;  and  (2)  the  orbital  surface  of  the  frontal 
lobe  resting  upon  the  floor  of  the  anterior  cranial  fossa.  The  latter  surface 
presents  near  its  medial  border  the  olfactory  sulcus,  a  straight,  deep  furrow, 
directed  rostrally  and  somewhat  medially,  that  lodges  the  olfactory  tract  and 
bulb.  To  its  medial  side  is  found  the  gyrus  rectus.  The  remainder  of  the 
orbital  surface  of  the  frontal  lobe  is  subdivided  by  irregular  orbital  sulci  into 

equally  irregular  orbital  gyri. 
16 


242 


THE    NERVOUS    SYSTEM 


From  the  foregoing  account  it  will  be  apparent  that  almost  the  entire  sur- 
face of  the  human  cerebral  hemisphere  is  formed  by  neopallium.  Of  the  parts 
already  described  only  the  uncus  and  adjacent  part  of  the  hippocampal  gyrus 
belong  to  the  archi pallium.  Other  superficial  portions  of  the  rhinencephalon, 
such  as  the  olfactory  bulb,  tract  and  trigone,  and  the  anterior  perforated  sub- 
stance, will  be  described  in  connection  with  the  hidden  parts  of  the  rhinen- 
cephalon in  Chapter  XVII. 

Suprasylvian  fissure 


Cerebral  hemisphere , 
Cerebellum 
Poslmedian  lobulet*-^ 
Ansiform  lobn 
Paraiiocculus\~ 
Paramedian  lobule* 
Flocculus1 

Chorioid  plexus  of 
fourth  ventricle 


Lateral  fissure 
, Insula 


Vin 

Olive        VII 
Trapezoid  body 


V   TV  I 


VI 


Pons 


fissure      fissure 
Mammillary  body 
Hippocampal  gyrus 
Cerebral  peiuncle 


»  Olfactory  bulb 

Lateral  olfactory  gyrus 


Fig.  173. — Lateral  view  of  the  sheep's  brain. 


The  surface  form  of  the  cerebral  hemisphere  of  the  sheep  is  illustrated  in 
Figs.  83.  84.  and  173.  On  these  figures  are  indicated  the  names  of  the  chief 
sulci  and  gyri.  It  will  be  of  interest  to  note  the  position  of  the  motor  cortex 
in  the  sheep  as  given  in  Fig.  82.  Since  this  corresponds  to  the  precentral  gyrus 
in  man,  it  will  be  seen  that  there  is  little  in  the  sheep's  brain  to  correspond  to  the 
ro.-tral  part  of  the  frontal  lobe  in  man. 


CHAPTER  XVI 

THE  INTERNAL  CONFIGURATION  OF  THE  CEREBRAL  HEMISPHERES 

Whin  a  horizontal  section  is  made  through  the  cerebral  hemisphere  at  the 
level  of  the  dorsal  border  of  the  corpus  callosum  the  central  white  substance 
will  be  displayed  in  its  maximum  extent  and  will  appear  as  a  solid,  semioval 
mass,  known  as  the  centrum  semiovaU  (Figs.  174,  175).  It  will  also  Ik-  apparent 
that  lamella'  extend  from  this  central  white  substance  to  form  the  medullary 
centers  of  the  various  convolutions,  and  that  over  this  entire  mass  the  cortex  is 
spread  in  an  uneven  layer,  thicker  over  the  summit  of  a  convolution  than  at 
the  bottom  of  a  sulcus.  This  medullary  substance  is  composed  of  three  kinds 
of  fillers:  (1)  fibers  from  the  corpus  callosum  and  other  commissures  joining  the 
cortex  of  one  hemisphere  with  that  of  the  other;  (2)  fibers  from  the  internal  cap- 
sule, uniting  the  cortex  with  the  thalamus  and  lower  lying  centers;  and  (3) 
fibers  running  from  one  part  of  the  cortex  to  another  within  the  same  hemi- 
sphere (p.  296). 

The  Corpus  Callosum. — At  the  bottom  of  the  longitudinal  fissure  of  the 
cerebrum  is  a  broad  white  band  of  commissural  fibers,  known  as  the  corpus 
callosum,  which  connects  the  neopallium  of  the  two  hemispheres.  While  the 
medial  portion  of  this  commissure  is  exposed  in  the  floor  of  the  longitudinal 
fissure,  its  greater  part  is  concealed  in  the  white  center  of  the  hemisphere  where 
its  fibers  radiate  to  all  parts  of  the  neopallium,  forming  the  radiation  of  the 
corpus  callosum.  When  examined  in  a  median  sagittal  section  of  the  brain  the 
corpus  callosum  is  seen  to  be  arched  dorsally  and  to  be  related  on  its  ventral 
surface  to  the  fornix  and  septum  pellucidum  (Figs.  84,  158,  170).  The  latter 
consists  of  two  thin  membranous  plates,  stretched  between  the  corpus  callosum 
and  the  fornix  and  separated  by  a  narrow  cleft-like  space,  the  cavum  septi 
pellucidi  (Fig.  177).  If  the  septum  has  been  torn  away  it  will  be  possible  to 
look  into  the  lateral  ventricle  and  see  that  the  corpus  callosum  forms  the  roof 
of  a  large  part  of  that  cavity.  At  its  rostral  extremity  it  curves  abruptly  toward 
the  base  of  the  brain,  forming  the  genu,  and  then  tapers  rapidly  to  form  the 
rostrum.  The  latter  is  triangular  in  cross-section,  with  its  edge  directed  toward 
the  anterior  commissure  to  which  it  is  connected  by  the  rostral  lamina.     The 

243 


244 


THE    NERVOUS    SYSTEM 


body  of  the  corpus  callosum  (truncus  corporis  callosi),  arching  somewhat  dor- 
sally,  extends  toward  the  occiput  and  terminates  in  the  splenium,  a  thickened 
rounded  border  situated  dorsal  to  the  pineal  body  and  corpora  quadrigemina. 
Related  to  the  concave  or  ventral  side  of  the  corpus  callosum  are  the  fornix, 
septum  pellucidum,  lateral  ventricles,  tela  chorioidea  of  the  third  ventricle,  and 
the  pineal  body  (Fig.  170). 


Genu  of  corpus  callosum 


Cingulum  (cut) 


Centrum  semi-. 

Medial  I  ■ 

tudinal  stria    \ 


Cingulum  (cut 


Splenium  of 
corp.  callosum 


Frontal  part  of 
,-    radiation  of 
corp.  callosum 

Intersection  of 
fibers  from  cor- 
_.  pus  callosum 
and  corona 
radiata 
__  Superior  longi- 
tudinal fas- 
ciculus 
~ '  Temporal  lobe 

~:--Insula 

Radiation  of 
"  corp.  callosum 

^JTransverse  tem- 
poral gyri 


•Optic  radiation 


'--^-Tapctum 


Occipital  part  of 
"  radiation  of 
corp.  callosum 


Fig.  174. 


-Dissection  of  the  human  telencephalo 

Dorsal  view. 


show  the  radiation  of  the  corpus  callosum. 


Turning  again  to  the  dorsal  aspect  of  the  corpus  callosum.  a  careful  inspec- 
tion will  show  that  at  the  bottom  of  the  great  longitudinal  fissure  it  is  covered 
by  a  very  thin  coating  of  gray  matter,  continuous  with  the  cerebral  cortex  in 
the  depths  of  the  sulcus  of  the  corpus  callosum  (Figs.  174.  175).  This  is  a  rudi- 
mentary portion  of  the  hippocampus  and  is  known  as  the  supracallosal  gyrus  or 
indusium  griseum.     In  this  gray  band  there  are  embedded  delicate  longitudinal 


mi     IMI  RNAI   «  ONI  K.i  RATIO*!    OP    mi.    I  I  ii  BRAL    BEMISPH1  I  245 

strands  of  nerve  fibers.  Two  of  these,  placed  close  together  on  either  side  of 
the  median  plain',  are  known  as  the  medial  longitudinal  stria.  Further  lateral- 
ward  on  cither  side,  hidden  within  the  sulcus  of  the  corpus  callosum,  Is  a  less 
well  developed  band,  the  lateral  longitudinal  stria. 

The  corpus  callosum  is  transversely  striated  and  is  composed  of  fibers  thai 
pass  from  one  hemisphere  to  the  other.     By  dissection  these  may  be  foil* 
into  the  centrum  semiovale,  where  they  constitute  the  radiation  of  the  corpus 


Genu  of  corpus  callosum 
;    Medial  longitudinal  stria 


Hippocampal  rudiment 
\  -Body  of  corpus  callosum 

Radiation  of  corpus  callosum 
-  Corona  radiata 

Intersection  of  corona  ra- 
diata and  radiation  of 
corpus  callosum 


Lateral  longitudinal  stria 
Splenium  of  corpus  callosum 

Fig.  175. — Dissection  of  the  telencephalon  of  the  sheep  to  show  the  radiation  of  the  corpus  cal- 
losum.   Dorsal  view. 


callosum  and  intersect  those  from  the  internal  capsule  in  the  corona  radiata 
(Figs.  174,  175).  The  fibers  of  the  genu  sweep  forward  into  the  frontal  lobe, 
constituting  the  frontal  part  of  the  radiation.  Fibers  from  the  splenium  bend 
backward  toward  the  occipital  pole,  forming  the  occipital  part  of  the  radiation 
or  forceps  major.  In  the  human  brain  fibers  from  the  body  and  splenium 
of  the  corpus  callosum  sweep  outward  over  the  lateral  ventricle,  forming  the 
roof  and  lateral  wall  of  its  posterior  horn  and  the  lateral  wall  of  its  inferior 
cornu.     Here  they  constitute  a  very  definite  stratum  called  the  tapctum. 


246 


THE  NERVOUS  SYSTEM 
THE  LATERAL  VENTRICLE 


When  the  corpus  callosum  and  its  radiation  arc  cut  away  a  cavity,  known 
as  the  lateral  ventricle,  is  uncovered.  It  is  lined  by  ependyma,  continuous  with 
the  ependyma]  lining  of  the  third  ventricle  by  way  of  the  interventricular  for- 
amen. This  cavity,  which  contains  cerebrospinal  fluid,  varies  in  size  in  differ- 
ent parts,  and  in  some  places  is  reduced  to  a  mere  cleft  between  closely  apposed 
walls.  The  shape  of  the  ventricle  is  highly  irregular  (Fig.  176).  As  constit- 
uent parts  we  recognize  a  central  portion,  anterior  and  inferior  horns,  and  in 
man  also  a  posterior  horn.  The  latter  part  develops  rather  late  in  the  human 
fetus  as  a  diverticulum  from  the  main  cavity. 


Third  ventricle 


-Ant.  horn 
Centra',  part        \Latcral  vn"n 
Inf.  horn' 


Q  viral  pari 


Ant.  horn 


Fourth  ventricle 


Fourth  ventricle 


v  Interventricular  for. 
\f '  Optic  recess 
UJ\\  '  Infundibulum 

\  \  \  '  Third  ventricle 
\  \  ^  Inf.  horn 
K\*  Suprapineal  recess 
^  Cerebral  aqueduct 

lateral  recess 


B 


Fig.  176. — Two  views  of  the  brain  ventricles  of  man:  A,  Dorsal  view;  B,  lateral  view. 

The  anterior  horn,  or  cornu  anterius,  is  the  part  which  lies  rostral  to  the 
interventricular  foramen.  Its  roof  and  rostral  boundary  are  formed  by  the 
corpus  callosum.  Its  medial  wall  is  vertical  and  is  formed  by  the  septum  pellu- 
cidum,  which  is  stretched  between  the  corpus  callosum  and  the  fornix  (Figs. 
177,  178).  The  sloping  floor  is  at  the  same  time  the  lateral  wall,  and  is  formed 
by  the  head  of  the  caudate  nucleus,  which  bulges  into  the  ventricle  from  the 
ventrolateral  side.  In  frontal  section  the  cavity  has  a  triangular  outline;  and 
in  such  a  section  its  walls  and  the  relation  which  the)'  bear  to  the  rest  of  the  brain 
can  be  studied  to  advantage  (Fig.  186). 

The  central  part  or  body  of  the  lateral  ventricle  extends  from  the  inter- 
ventricular foramen  to  the  splenium  of  the  corpus  callosum,  where  in  man  the 
cavity  bifurcates  into  posterior  and  inferior  horns.     The  roof  of  the  central 


nil     IMU'WI     CONFIGURATION  O]     mi     CEREBRAL  HEMISPHE]  j.47 

part  is  formed  1>\  the  corpus  callosum,  and  the  medial  wall  by  the  septum  pellu- 
ddum.  The  floor,  which  slants  to  meet  the  roof  al  the  lateral  angle,  is  com 
posed  l'n>m  within  outward  of  the  following  structures:  the  fornix,  chorioid 
plexus,  lateral  part  of  the  dorsal  surface  of  the  thalamus  in  man.  but  not 
in  the  sheep),  the  stria  terminalis,  vena  terminalis,  and  the  caudate  nucleus 
(Figs.  177  180,  188).  The  caudate  nucleus  tapers  rapidly  as  it  Is  folli 
from  tin-  anterior  horn  into  the  body  of  the  ventricle  (Fig.  177  .     The  cavity 


Longitudinal  fissure  of  cerebrum  ■""  "/  corPu 

ina  of  septum  peUucidum  Corpus  callosum 

umn  of  fornix       v^^HM^  i    ^Wl^h-^  /  Cavity  of  u  plum  pel!,,,  i 


Column  of  fornix 
Caudate  nucleus 


Interventricular  foramen 


Thalamus 


Body  of  fornix  ■ 


Chorioid  plexus 


Transverse  fissure  of 
cerebrum 


•  septum  peUucidum 
Anterior  horn  of  lateral  ventricle 
audate  nucleus 

Chorioid  plexus  of  lateral 
ventricle 

Terminal  stria 

C>  ntral  portion  of 
lateral  ventricle 


Chcrioid  glomus 


J^-Crus  of  fornix 

^Inferior  horn  of 
lateral  ventricle 


S pi 'en 1 urn  of  corpus  callosum 


/'    :>rior  horn  of  lateral  ventricle 
Calcarine  fissure 

Cerebellum 


Fig.  177  — Dissection  of  the  human  telencephalon.     The  corpus  callosum  has  been  partly  removed, 
and  the  lateral  ventricles  have  been  exposed.     Dorsal  view.     (Sobotta-Mc  Murrich.) 


is  lined  throughout  by  an  ependymal  epithelium,  indicated  in  red  in  Fig.  155. 
Between  the  caudate  nucleus  and  the  fornix  this  layer  of  ependyma  constitutes 
the  entire  thickness  of  the  wall  of  the  hemisphere.  In  man.  where  the  fornix 
and  caudate  nucleus  are  more  widely  separated  than  in  the  sheep,  this  epithelial 
membrane  rests  upon  the  thalamus  and  becomes  adherent  to  it  as  the  lamina 
affixa  (Figs.  154.  155).  At  the  margin  of  the  fornix  a  vascular  network  from  the 
tela  chorioidca,  i.  c,  from  the  pia  mater  in  the  transverse  cerebral  hs>ure.  i- 


248 


THE    NERVOUS    SYSTEM 


invaginated  into  the  ventricle,  pushing  this  epithelial  layer  before  it  and  con- 
stituting the  chorioid  plexus. 

The  posterior  horn,  or  cornu  posterius,  extends  into  the  occipital  lobe  of 
the  human  brain,  tapering  to  a  point,  and  describing  a  gentle  curve  with  con- 
cavity directed  medially  (Figs.  177,  181). 

The  tapetum  of  the  corpus  callosum  forms  a  thin  but  distinct  layer  in  the 
roof  and  lateral  wall  of  the  posterior  horn,  and  is  covered  in  turn  by  a  thicker 
layer  of  fibers  belonging  to  optic  radiation  or  radiatio  occipitothalamica  (Fig. 
190).     In  the  medial  wall  two  longitudinal  elevations  may  be  seen.     Of  these, 


Corpus  callosum  -. 


Head  of  fa  m!  a  If 
initials  " 


Body  of  fornix  -. 


Fimbria  of  hippo- 
campus ' 

Hippocampus  - 


Splcnium  of  corpus 
callosum 


--  -Genu  of  corpus 
callosum 

.  —  A  nterior  horn  of 
lateral  ventricle 

■  -  Thick    portion    of 
septum  pell  tie  id  um 

~~  Lateral  fissure 


Interventricular 
foramen 

Lateral  ventricle 


Fig.   178. — Dissection  of  the  telencephalon  of  the  sheep  to  show  the  lateral  ventricle  and  the 
structures  which  form  its  floor.     Dorsal  view. 


the  more  dorsal  one  is  known  as  the  bull  of  the  posterior  horn  (bulbus  cornu), 
and  is  formed  by  the  occipital  portion  of  the  radiation  of  the  corpus  callosum 
or  forceps  major.  The  other  elevation,  known  as  the  ealcar  avis,  is  larger  and 
is  produced  by  the  rostral  part  of  the  calcarine  fissure,  which  here  causes  a  fold- 
ing of  the  entire  thickness  of  the  pallium  (p.  238). 

The  inferior  horn,  or  cornu  inferius,  curves  ventrally  and  then  rostrally  into 
the  temporal  lobe  (Fig.  181).  The  angle  between  the  diverging  inferior  and 
posterior  horns  is  known  as  the  collateral  trigone.  This  horn  lies  in  the  medial 
part  of  the  temporal  lobe  and  does  not  quite  reach  the  temporal  pole.     The  roof 


Ill  I :    IMKkNAI.    CONFIGURATION    OF    THE    CEREBRAL    BEMISPHERES 


is  formed  by  the  while  substance  of  the  hemisphere,  and  along  ii  3  medial  bord<  r 
are  the  stria  terminalis  and  tail  of  the  caudate  nucleus.    At  the  end  of  the  latter 


{'iDiii  of  <  or  pus 
caUosurn" 

Septum  pell ik 'ilium  -  . 

Thick  portion  of  sip        I  ) 

turn  pellut  idiim'"^"  --^--- 

/ 

Hippocampus  -• — y s^T~"    //    /' 

Inferior   horn    of. 

lateral  ventricle    ^^^^^^H_ 

Transverse  fissure  of  cerebrum  ' 
Thalamus 

Fig.  179. 


Lateral  ventricle*-*. 


Head  of  caudate 

nth  h  us 

--/nli  rn  utricular 
foramen 


Churioiil  fissure 


-Fimbria  of 

hippocampus 


\  Hippocampal  commis  uri 
Pineal  body 


Septum  pcllucidum 


Thick  portion  of 
septum  pcllucidum 


Column  of  fornix 


Genu  of  corpus 

call  os  urn 


-    Head  of  caudate 
nucleus 


-  Interventricular 
foramen 

V Fimbria  of  hippo- 
campus 

-Inferior    horn    of 
lateral  ventricle 


Thalamus 


i  Thai  a  m  us  **  Hippoca  mpus 

_, .  ,        .  .  ,     /  >    Tcenia  of  thalamus 

Third  ventricle  !  !       ,        ,  "       . 

Pineal  bodv  Habcuular  trigone 


Fig.  180. 
Figs.  179  and  180.— Dissections  of  the  rostral  part  of  the  sheep's  brain  to  show  the  relation 
of  the  lateral  ventricles,  fornix,  fimbria,  and  hippocampus  to  the  transverse  fissure,  thalamus,  and 
third  ventricle.     Dorsal  views.     In  Fig.  180  a  triangular  piece,  including  portions  of  the  fornix, 
fimbria,  and  hippocampus,  has  been  removed. 

the  amygdaloid  nucleus  bulges  into  the  terminal  part  of  the  inferior  horn  (Fig. 
185).     The  floor  and  medial  wall  of  the  inferior  horn  are  formed  in  large  part 


25° 


THE    NERVOUS    SYSTEM 


by  the  following  structures,  named  in  their  order  from  within  outward:  the 
fimbria,  hippocampus,  and  (in  man)  the  collateral  eminence  (Figs.  181,  182, 
189).  Upon  the  fimbria  and  hippocampus  there  is  superimposed  the  chorioid 
plexus  (Fig.  183).  The  hippocampus  is  a  long,  prominent,  curved  elevation, 
with  whose  medial  border  there  is  associated  a  band  of  fibers,  representing  a 
continuation  of  the  fornix  and  known  as  the  fimbria.     These  parts  will  be  de- 


Lamina  of  septum  pellucidum 

Columns  of  fornix 
Anterior  tubercle  of  thai  a-    /fp 


Uncus , 


Ilippocampal 

(limitations 


Hippocarnpa 

gyrus  1 

Collateral  eminence'-^ 
Fimbria  of  hippo- 
campus . 
Collateral  trigone 
Posterior  commissure 

Hippocampus 

Calcar  avi 


Longitudinal  fissure  of  cerebrum 
Corpus  callosum 

Cavity  of  septum  pellucidum 
Interventricular  foramen 
Anterior  horn  of  lateral  ventricle 
Head  of  caudate  nucleus 

Jlassa  intermedia 
Third  ventricle 
■  Habenular  commissure 

.-  Habenular  trigone 

A  Inferior  horn  of  lateral 
ventricle 


,■■  Posterior  horn  of  lat- 
eral ventricle 


Pineal  body 


Posterior  horn  of  lateral  ventricle        "^-I___ 

i  Corpora  quadrigemina 

Vermis  of  cerebellum 

Fig.  181. — Dissection  of  the  human  brain  to  show  the  posterior  and  inferior  horns  of  the  lateral 
ventricle.  The  body  and  splenium  of  the  corpus  callosum  have  been  removed,  as  have  also  the  body 
of  the  fornix  and  the  tela  chorioidea  of  the  third  ventricle.  A  sound  has  been  passed  through  the 
interventricular  foramina.     Dorsal  view.     (Sobotta-McMurrich.) 

scribed  in  connection  with  the  rhinencephalon.     The  collateral  eminence  is  an 
elevation  in  the  lateral  part  of  the  floor  produced  by  the  collateral  fissure. 

The  thin  epithelial  membrane,  described  above  as  joining  the  edge  of  the 
fornix  with  the  caudate  nucleus  (Fig.  155),  continues  to  unite  these  structures  as 
they  both  curve  downward,  the  former  in  the  floor,  the  latter  in  the  roof,  of  the 
inferior  horn.     A  vascular  plexus  from  the  pia  mater  is  invaginated  into  the 


\ 


THE    [NTERNAL   CONFIGURATION    OF    THE    CEREBRAL    HEMISPHERES 

lateral  ventricle  along  this  curved  line,  carrying  before  it  an  epithalial  covering 
from  this  thin  membrane.  In  this  way  there  is  formed  the  chorioid  plexus  of  the 
lateral  ventricle  (Figs.  183,  184).  The  line  along  which  this  invagination  oc<  unci 
is  the  chorioid  fissure;  and  when  the  plexus  is  torn  away,  the  position  of  the 


Lateral  ventricle 


\ 


Interventrii  ular  foramen 


Hippocampus        / 
Fimbria  of  hippocampus' 

Body  of  fornix 

Optic  tract    1 

Internal  capsule 


/    f        |  Olfactory  bulb 

I     !         I  Rhinoccsle 

>  Genu  of  corpus  callosum 

I     Body  of  corpus  callosum 
Septum  pcllucidum 


Fig.    182. — Dissection  of  the  cerebral  hemisphere  of  the  sheep  to  show  the  lateral  ventricle. 

Lateral  view. 


fissure  is  indicated  by  an  artificial  cleft  extending  into  the  ventricle,  which  be- 
gins at  the  interventricular  foramen  and  follows  the  fornix  and  fimbria  in  an 
arched  course  into  the  temporal  lobe  (Fig.  205) . 


Hippocampus  Chorioid  plexus  of  lateral  ventricle 


Fig.  183.— Outline  drawing  from  Fig.  182,  to  show  the  location  of  the  chorioid  plexus  of  the  lateral 

ventricle. 


The  chorioid  plexus  of  the  lateral  ventricle  (Figs.  183,  184.  188)  is  continuous 
with  that  of  the  third  ventricle  at  the  interventricular  foramen,  from  which 
point  it  can  be  followed  backward  through  the  central  part  into  the  inferior 
horn.  It  is  coextensive  with  the  chorioid  fissure  and  is  not  found  in  the  anterior 
or  posterior  horns.     It  consists  of  a  vascular  network  derived  from  the  pia 


2«C2 


THE    NERVOUS    SYS1  EM 


mater,  and  especially  from  that  part  of  it  enclosed  in  the  transverse  fissure  and 
known  as  the  tela  chorioidea  of  the  third  ventricle.     It  is  covered  throughout 

Body  of  corpus  callosum 
.Lamina  of  septum  pdlucidum 


Longitudinal  fissun  of  cerebrum 


Anterior  horn  of  lateral 
ventricle 


Corpus  striatum 
Interventricular  for. 


Columns  of  fornix 

C(  ntral  portion  of 

lateral  ventricle 

Internal  cerebral 

veins 

Clwrioid  vein 

Chorioid  arlcrv 


Inferior    horn    of 
lateral  ventricle 


Collateral  trigone 

,• 
Posterior  horn  "■ 


Calcar  a\ 


Great  cerebral  vein 


.,   -Cavity  of  sept,  pdlucidum 

Lamina  of  septum 
S  pellucid  urn 

]'<  in  of  septum 
pellucid  um 

-  Terminal  vein 
'      Thalamus 


-■Corpus  striatum 
Lateral  chorioid 

■  plexus 

■  Tela  chorioidea 
of  third  ventricle 

Chorioid  glomus 


Hippocampal     Body  of  corpus    Body  of  fornix   Crura  of  fornix 
com  missure  callosum 


Fig.  184. — Dissection  of  the  human  brain  to  show  the  tela  chorioidea  of  the  third  ventricle 
and  the  hippocampal  commissure.  The  body  of  the  corpus  callosum  and  the  fornix  have  been 
divided  and  reflected.  Dorsal  view,  except  that  the  ventral  surfaces  of  the  reflected  corpus 
callosum  and  hippocampal  commissure  are  seen.      (Sobotta-McMurrich.) 

by  a  layer  of  epithelium  of  ependymal  origin,  which  is  adapted  to  every  uneven- 
ness  of  its  surface  (Fig.  155). 


THE  BASAL  GANGLIA  OF  THE  TELENCEPHALON 
There  are  four  deeply  placed  masses  of  gray  matter  within  the  hemisphere, 
known  as  the  caudate,  lentijorm  and  amygdaloid  nuclei,  and  the  claustrum.     The 


I  III      [NTERNAL  CONFIGURATION   01      nil     CEREBRAL  HEMISPHERES 


253 


two  former,  together  with  the  white  fascicles  of  the  internal  capsule  which 
separate  them,  constitute  the  corpus  striatum  (Fig.  185 

I  he  caudate  nucleus  (nucleus  caudatus)  is  an  elongated  mass  of  gray  matter 
bent  "ii  itself  like  a  horseshoe,  and  is  throughout  it-  entire  extent  closely  re 


Caudate  nucleus 


Thalamus 


Lenticular  nucleus 
Amygdaloid  nucleus 
Caudate  nucleus 
Thalamus 

Tail  of  caudate  nucleus 

Internal  capsule 
Lenticular  nucleus 

Caudate  nucleus 

Thalamus 


f\ Tail  of  caudate  nucleus 

Internal  capsule 


Fig.  185. — Diagrams  of  lateral  view  and  sections  of  the  nuclei  of  the  corpus  striatum  with  the 
internal  capsule  omitted.  A  and  B  below  represent  horizontal  sections  along  the  lines  A  and  B 
in  the  figure  above.  The  figure  also  shows  the  relative  position  of  the  thalamus  and  the  amygda- 
loid nucleus.      (Jackson-Morris.) 

lated  to  the  lateral  ventricle  (Figs.  91,  177,  178,  186.  187,  188,  191).  Its  swol- 
len rostral  extremity  or  head  is  pear  shaped  and  bulges  into  the  anterior  horn  of 
the  lateral  ventricle.  The  remainder  of  the  nucleus  is  drawn  out  into  a  long, 
slender,  highly  arched  tail.  In  the  floor  of  the  central  part  of  the  ventricle  the 
head  gradually  tapers  off  into  the  tail,  which  finally  curves  around  into  the  roof 


2  54 


THE   NERVOUS    SYSTEM 


of  the  inferior  horn  and  extends  rostrally  as  far  as  the  amygdaloid  nucleus. 
Because  of  its  arched  form  it  will  be  cut  twice  in  any  horizontal  section  which 
passes  through  the  main  mass  of  the  corpus  striatum,  and  in  any  frontal  section 
through  that  body  behind  the  amygdaloid  nucleus  (Figs.  185,  189,  191).  The 
head  of  the  caudate  nucleus  is  directly  continuous  with  the  anterior  perforated 
substance;  and  ventral  to  the  anterior  limb  of  the  internal  capsule  it  is  fused  with 
the  lentiform  nucleus  (Fig.  186). 

The  lentiform  or  lenticular  nucleus  (nucleus  lentiformis)  is  deeply  placed 
in  the  white  center  of  the  hemisphere  and  intervenes  between  the  insula,  on  the 


Stria 
longitu 

diu3lis  I  lateralis 


Nucleus  lentifor 
mis  (Putamen 


Polus  temporalis 


Fissura  longitiuli 

nalis  cerebri 
..Gyrus  cinguli 

Sulcus  corporis 
callosi 


Cornu  anterius 
ventriculi 
lateralis 
.Vena,  septi 
pe'.tuciili 

_    Septum 

pellucidum 
-.Fissura  cerebri 

lateralis(Sylvii) 

vlnsula 
Rostrum  cor- 
poris callosi 

.  Gyrus  sub- 
callosus 


Area  parolfac- 
toria  (Brocae) 
^Fissura  cerebri 
lateralis  (Sylvii) 


Fig.  186. — Frontal  section  of  the  human  brain  through  the  rostral  end  of  the  corpus  striatum  and 
the  rostrum  of  the  corpus  callosum.      (Toldt.) 


one  hand,  and  the  caudate  nucleus  and  thalamus  on  the  other  (Figs.  185,  191, 
194).  In  shape  it  bears  some  resemblance  to  a  biconvex  lens.  Its  lateral, 
moderately  convex  surface  is  nearly  coextensive  with  the  insula  from  which  it 
is  separated  by  the  claustrum.  Its  ventral  surface  rests  upon  the  anterior  per- 
forated substance  and  the  white  matter  forming  the  roof  of  the  inferior  horn  of 
the  lateral  ventricle  (Figs.  187-189).  Its  sloping  medial  surface  is  closely 
applied  to  the  internal  capsule.  The  lentiform  nucleus  is  not  a  homogeneous 
mass,  but  is  divided  into  three  zones  by  internal  and  external  medullary  lamina. 
The  most  lateral  zone  is  the  largest  and  is  known  as  the  putamen.  The  two 
medial  zones  together  form  the  globus  pallidus. 


THE    INIIkWI.    CONFIGURATION    OF     Mil.    CEREBRAL    HEMISPHERES 


255 


(facleu  1  caudatus 
1  lapul 


Capsula  intei  na 
(Pars  i: 


tormina 

Foramen  inters' 

vcntriculare 

(Monrol 

Substantia  per-  , 

forata  anterior 

Uncus 


idinalia 

cet- 

tllotum 

mterius 
ricull 

laterlais 

'triculi 
Intel  1 

ptum  pellu- 
cidum 


una 
fornicis 

Fissura  cerebri 

lateral 

*-"  Gyri  insulae 

_  Recessus  opti- 
cus vcntriculi 

terlii 
Tractus  opticus 

Chiasma  opti- 
■  cum  (po 
part; 

N  Commissure  inferior 


Fig.  187. — Frontal  section  of  the  human  brain  through  the  anterior  commissure.     (Toldt.) 


Ventriculus  lateralis 
(Pars  centralis 

Plexus  chorioideus^   ,. 
vcntriculi  lateralis  ^ 

Nucleus    ,\ 

caudatus~"\- 

Massa  inter-.    -. 

media 

Capsula  interna--, 

iPutamen-Jiii 
Globus=; 
pallidus 
Capsula  externa.. 

Claustrum.^- 
Ansa  peduncu- 

laris 
Tractus  opticus- — - 
Pedunculus.  tha -„--' 
lami  inferior 

Cornu  inferius  ve:  -x" 
triculi  lateralis 

Digitationes;-"  :\ 
hippocampi    N 
N.  oculomotorius'' 


Corpus  callosum 


Ventriculus 
tertius 

Thalamus 

Fasciculus 
-;'    \      ^.J--  thalamo- 
mamillaris 

—  Ansa  lenticularis 

.Nucleus  hypo- 
thalamics 
(Corpus  Luysi) 

Substantia 
-  nigra 

- — Basis  pedunculi 

Corpus 
'•■-  mamillare 

I      Fossa  inter- 
peduncularis 

-Pons  (Varoli) 


Fig.  188. — Frontal  section  of  the  human  brain  through  the  mammillary  bodies.     (Toldt.) 


The  putamen  is  larger  than  the  globus  pallidus  and  is  encountered  alone  in 
frontal  sections  through  either  the  rostral  or  caudal  extremities  of  the  corpus 
striatum  (Fig.  189),  and  also  in  horizontal  sections  above  the  level  of  the  globus 


2^6 


THE    NERVOUS    SYSTEM 


pallidus  (Fig.  191).  It  is  fused  rostrally  with  the  caudate  nucleus,  which  it 
resembles  in  color  and  structure. 

The  globus  pallidus  is  lighter  in  color  and  is  subdivided  into  two  parts,  of 
which  the  medial  is  the  smaller.  Both  parts  are  traversed  by  many  fine 
white    fascicles   from   the  medullary  laminae. 

Especially  in  the  anterior  part  of  the  internal  capsule  bands  of  gray  sub- 
stance stretch  across  from  the  lentiform  to  the  caudate  nucleus,  producing  a 
striated  appearance  (Fig.  187).  This  appearance,  which  is  accentuated  by  the 
medullarv  laminae  and  the  liner  fiber  bundles  in  the  lentiform  nucleus,  makes 


Tela  chorioidea^ 
ventriculi  tertii    N^tf  ' 

Cauda  nuclei 
caudati 

Thalamus » 
Capsula  interna-^ 

Putamen,  _.•• 

- 

Claustrum-..':. 

Nucleus 
habenulae 
Cauda  nuclei 
caudati  ■ 
Tractus  opticus- 
Fimbria  hippo- 
campi 

Fascia  dentata-' 
hippocampi 

Pedunculus  cerebri 


V  cerebri  interna 

Plexus  chorio- 
ideus  ventriculi 

tertii 
Commissura 
•  -'  habenularum 
"     Commissura 
V      posterior 

Aditus  ad  aquae- 
srf*    ductum  cerebri 

Fasciculus  retro- 

""  flcxus(Mcynerti) 

_  Cornu  inferius 
ventriculi 
lateralis 

--...Nucleus  ruber 

---  Nucleus  bypo- 
thalamicus 
(Corpus  Luysi) 


x- Substantia  nigra 
"^Pons  (Varoli) 


Recessus  posterior  fossae  interpeduncularis'' 
Fig.  189. — Frontal  section  of  the  human  brain  through  the  rostral  part  of  the  pons.     (Toldt.) 


the  term  corpus  striatum  an  appropriate  name  to  apply  to  the  two  nuclei  and 
the  internal  capsule,  which  separates  them. 

The  claustrum  is  a  thin  plate  of  gray  substance,  which,  along  with  the  white 
matter  in  which  it  is  embedded,  separates  the  putamen  from  the  cortex  of  the 
insula.  Its  lateral  surface  is  somewhat  irregular,  being  adapted  to  the  convolu- 
tions of  the  insula,  with  which  it  is  coextensive  (Figs.  188,  191).  Its  concave 
medial  surface  is  separated  from  the  putamen  by  a  thin  lamina  of  white  matter, 
known  as  the  external  capsule.  By  some  authorities  the  claustrum  is  thought 
to  be  a  detached  portion  of  the  lentiform  nucleus,  while  others  believe  that  it 
ha-  been  split  off  from  the  insular  cortex.  It  is  probable  that  neither  of  these 
views  is  strictly  correct.     However,   according  to  the  recent  work  of  Elliot 


[HE    INTERNAL  CONFIGURATION    OP     nil     CEREBRA1     II     MISPHERES 


257 


Smith  (1919),  the  claustrum,  putamen,  amygdaloid  nucleus,  and  the  greater 
part  of  the  caudate  nucleus  are  pallia!  derivatives  and  arc  closelj  related  mor 
phologicaUy  to  the  neopallium;  while  the  globus  pallidus  is  the  representative 

in  the  mammalian  brain  i)\   the  Corpus  Striatum  of  lower  forms,  a-  seen  in  the 
shark   (Fig.  9). 

The  Amygdaloid  Nucleus.     In  the  roof  of  the  terminal  part  of  the  inferior 
ventricular  horn,  at  the  point  where  the  tail  of  the  caudate  nucleus  ends,  there 

is  located  a  small  mass  of  gray  matter,  known  as  the  amygdaloid  nucleus  (Fig. 


Radiatio  corporis 
callosi 


Hippocampus 

Corpora 

quadrigemina  ■ 
Nucleus 
colliculi.^ 
inferioris 

Aquaeductus___'_ 
cerebri 

Nucleus  n 

trochlearis 
Fasciculus 
longitudinalis 
medialis 

Cerebellum. - 
Brachium  pontis  -- 

Flocculus- — 
Pyramis  medullae  oblongatae 


Splenium  cor- 
poris callosi 

Tela  chorio- 

idea  ven- 
triculi  tcrtii 
Corpus 
-''  pineale 

<  ornu  poste- 

.-'  riiis-ventri- 

culi  lateralis 

Glomus 

.  -  '  \       chorioideum 

-  Tapetum 

Radiatio  occi- 
pitothalamica 
--  Eminentia 
collatcralis 

Fissura 
collateralis 

Lemniscus 

lateralis 
-Brachium  con- 

junctivum 
-Stratum  griseum 
centrale 
--Lemniscus  medialis 


-  N.  vagus 


Fig.  190. — Frontal  section  of  the  human  brain  through  the  splenium  of  the  corpus  callosum.     View 
into  the  posterior  horn  of  the  lateral  ventricle.     (Toldt.) 

185").     It  is  continuous  with  the  cerebral  cortex  of  the  temporal  lobe  lateral  to 
the  anterior  perforated  substance  (Fig.  198;  Landau.  1919). 

The  external  capsule  is  a  thin  lamina  of  white  matter  separating  the  claus- 
trum from  the  putamen.  Along  with  the  internal  capsule  it  encloses  the  lenti- 
form  nucleus  with  a  coating  of  white  substance. 

THE  INTERNAL  CAPSULE 

The  internal  capsule  is  a  broad  band  of  white  substance  separating  the 
lentiform  nucleus  on  the  lateral  side  from  the  caudate  nucleus  and  thalamus  on 
the  medial  side  (Figs.  191,  192).     In  a  horizontal  section  through  the  middle 


K 


-^ 


THE    NERVOUS    SYSTEM 


of  the  corpus  striatum  it  has  the  shape  of  a  wide  open  V.     The  angle,  situated 
in  the  interval  between  the  caudate  nucleus  and  the  thalamus,  is  known  as  the 


Truncus  corporis  callosi 
Septum  pellucidum 
Corpus  fornicis 


Comu  inferius 
ventriculi 
lateralis 

Glomus  chorio- 
ideum 


Radiatio  occi- 
pitothalamica 
(Gratioleti) 


,  Genu  corporis  callosi 

,Cornu  anterius  ventriculi  lateralis 
uclei  caudati 
urr.na  fornicis 
Capsula  interna 

Insula 

, Capsula  externa 

t  Claustrum 

^Putameni 

Nucleus 
_.  .  }    lend- 

^Globus    I  fonnis 

pallidus  I 


Massa  inter- 
media 

Ventriculus 

tertius 
Stria  medullaris 
thai  ami 
-  Nucleus 
habenulae 

iK--Habenula 

S""""  Cauda  nuclei 
;^3       '     caudati 
jj"'  Fimbria  hippo- 
campi 

t^-Corpus  pineale 
Hippocampus 


Splenium  corporis  callo 


vEminentia 
colla'.eralis 


Calcar  avis 


Cornu  posterius 
ventriculi  lateralis 


Fissura  calcarina 


Fig.  191. — Horizontal  sections  of  the  human  brain  through  the  internal  capsule  and  corpus 
striatum.  The  section  on  the  right  side  was  made  1.5  cm.  farther  ventralward  than  that  on  the 
left.     (Toldt.) 


genu.  From  this  bend  the  frontal  part  or  anterior  limb  of  the  internal  capsule 
extends  laterally  and  rostrally  between  the  thalamus  and  the  head  of  the  caudate 
nucleus;  while  the  occipital  part  or  posterior  limb  of  the  internal  capsule  extends 


Till      IMKRNAL    CONFIGURATION    OF    Till      >  I   II   BRAL    III  M  I  - 1  •  1 1 1  II 


259 


lateralis  and  toward  the  occiput  between  the  lentiform  nucleu    and  the  thala 
mus. 

The  anterior  limb  of  the  internal  capsule,  intervening  between  the  caudate 
and  lentiform  nuclei,  is  broken  up  by  hand-  of  gray  matter  connecting  these 
two  nuclei.  It  consists  of  corticipetal  and  corticifuga]  fibers.  The  former 
belong  to  the  frontal  stalk  of  the  thalamus  or  anterior  thalamic  radiation  from  the 
lateral  nucleus  of  the  thalamus  to  the  cortex  of  the  frontal  lobe.     The-  cortici 


Septum  pettucidum  % 
Fornix^ 

Chorioid  fissur 

Third  ventricle-* 

Thalamus-. 

Habenular  trigone  .  , 

Habcnular  commis-  „  '_ 
sure  I 

Transverse  fissure  -*p 

Pineal  hotly  --  f- 

Inferior  horn  of 
lateral  ventricle 

Superior  colliculus 


-<„ 


Genu  of  corpus  callosum 

..'Anterior  horn  of  lateral  ventricle 

^..Anterior  limb  <f  internal  cap- 
sule 

Head  of  caudate  nucleus 

-  In 

External  <  apsule 

v  Lentiform  nucleus 

''Claustrum 

-.  Genu  of  internal 
capsule 
Posterior  limb  of 
internal  capsule 

Chorioid  fissure 
X    ■'   '"Fimbria  of  hippocampus 
\*        "Hippocampus 

-^Cerebellum 


Medulla  oblongata 


Fig.  192.— Horizontal  section  through  the  sheep's  brain,  passing  through  the  internal  capsule  and 

corpus  striatum. 

fugal  fibers  form  the  frontopontine  tract  from  the  cortex  of  the  frontal  lobe  to 
the  nuclei  pontis  (Fig.  193). 

The  posterior  limb  of  the  internal  capsule  intervenes  between  the  thalamus 
and  the  lentiform  nucleus,  and  bends  around  the  posterior  end  of  the  latter 
on  to  its  ventral  surface  (Fig.  194).  It  accordingly  consists  of  three  parts, 
designated  as  lenticulothalamic.  retrolenticular,  and  sublenticular.  The  lentic- 
ulothalamic  part  consists  of  fibers  belonging  to  the  thalamic  radiation  intermingled 
with  others  representing  the  great  efferent  tracts  which  descend  from  the  cere- 


260 


THE    NERVOUS    SYSTEM 


bra!  cortex  (Fig.  193).  Of  these,  the  corticobulbar  tract  to  the  motor  nuclei  of 
the  cranial  nerves  occupies  the  genu,  and  the  corticospinal  tract  the  adjacent 
portion  of  the  posterior  limb.     The  fibers  of  the  corticospinal  tract  are  so  ar- 


"^ -      -  Caudate  nucleus 

'Frontopontine  tract 
interior  thalamic  radiation 

iCorticobulbar  tract 

^'Globus  pallidas 

■  -Corticorubral  tract 
■Corticospinal  trad  (arm) 
■Corticospinal  tract  (leg) 

■  Pulamcn 

■  Thalamic  radiation  (sensory  fibers) 

Auditor  \  radiation 


Thalamus 


Optic  radiation 
Fig.  193. — Diagram  of  the  internal  capsule. 


ranged  that  those  for  the  innervation  of  the  arm  are  nearer  the  genu  than  those 
for  the  leg.  Accompanying  the  corticospinal  tract  are  descending  fibers  from 
the  cortex  of  the  frontal  lobe  to  the  red  nucleus,  the  corticorubral  tract.     Those 


Coronal  fibers  from  anterior 
limb  of  internal  capsule  ^ 

Lent/form  nucleus  "  " 


—Coronal  fibers  from  retro- 
lenticular  part  of  inter- 
nal capsule 


\Coronal  fibers  from  sublenticular  part 
of  internal  capsule 


Anterior  commissure- 
A  nsa  pedu  nail  oris ' 

Fig.   194! — The  lentiform  nucleus  and  the  corona  radiata  dissected  free  from  the  left  human 

cerebral  hemisphere.     Lateral  view. 


fibers  of  the  thalamic  radiation  which  run  to  the  posterior  central  gyrus  and  con- 
vey general  sensory  impulses  from  the  lateral  nucleus  of  the  thalamus  are  sit- 
uated behind  the  corticospinal  tract.     The  rctrolcnticular  part  of  the  internal 


THE    INTERNAL   CONFIGURATION    OF    [HE    CEREBRAL    HEMISPHERJ  26] 

capsule  rests  upon  the  lateral  surface  of  the  thalamus  behind  the  lentiform 
nucleus  and  contains:  (1)  the  optic  radiation  from  the  pulvinar  and  lateral 
geniculate  body  to  the  cortex  in  the  region  oi  the  calcarine  fissure,  and  (2)  the 
acoustic  radiation  from  the  medial  geniculate  bod}  to  the  tian -\er  e  temporal 
gyrus.  The  sublenticular  part  of  the  internal  capsule  lies  ventral  to  the  po  - 
terior  extremity  of  the  lenticular  nucleus  and  contains  the  temporopontine  tra<  I 
from  the  cortex  of  the  temporal  lobe  to  the  nuclei  pontis. 

Dissections  of  the  Internal  Capsule  (Figs.  87,  88,  91,  1<M.  195).  A  large 
part  of  the  libers  of  the  internal  capsule,  including  the  corticopontine,  cortico- 
bulbar,  and  corticospinal  tracts,  are  continued  as  a  broad  thick  strand  on  the 
ventral  surface  of  the  cerebral  peduncle,   with  which   we  are  already   familiar 


Radiation  of  corpus  col 
losutn forming  roof , 
of  lateral  ventricles    " 

Anterior  limb  of  inter- 
na! capsule  dan-  ^      .  -v 
dale  impression) 


Posterior  limb  of  internal  eapsule 
{thalamic  impression) 


/  Tapetum 


Frontal  pole 

Genu  internal  cap-  ~—>r-" 
sale 

Anterior  commissure 

Optic  tract  '' 
Temporal  lobe  ''  \ 

Basis  peduneuli 

Fig.  195. — Dissection  of  the  human  cerebral  hemisphere,  showing  the  internal  capsule  exposed 
from  the  medial  side.     The  caudate  nucleus  and  thalamus  have  been  removed. 


under  the  name  basis  peduneuli.  By  removing  the  optic  tract,  temporal  lobe, 
insula,  and  lentiform  nucleus  this  strand  can  easily  be  traced  into  the  internal 
capsule  where  it  is  joined  by  many  fibers  radiating  from  the  thalamus  and 
spreads  out  in  a  fan-shaped  manner  (Figs.  87,  88),  forming  a  curved  plate  which 
partially  encloses  the  lentiform  nucleus.  As  seen  from  the  lateral  side,  the  line 
along  which  the  libers  of  the  internal  capsule  emerge  from  behind  the  lentiform 
nucleus  forms  three-fourths  of  an  ellipse  (Fig.  194).  Beyond  the  lentiform  nu- 
cleus the  diverging  strands  from  the  internal  capsule,  known  as  the  corona 
radiate,  join  the  central  white  substance  of  the  hemisphere  and  intersect  with 
those  from  the  corpus  callosum  (Figs.  174,  238). 

An  instructive  view  of  the  internal  capsule  may  also  be  obtained  by  remov- 


262 


THE   NERVOUS   SYSTEM 


ing  the  thalamus  and  caudate  nucleus  from  its  medial  surface.  It  is  then  seen 
to  bear  the  imprint  of  both  of  these  nuclei,  and  especially  of  the  thalamus;  and 
between  the  two  impressions  it  presents  a  prominent  curved  ridge  (Fig.  195). 
This  ridge  is  responsible  for  the  sharp  bend  known  as  the  genu,  which  is  evi- 
dent in  horizontal  sections  at  appropriate  levels  through  the  capsule.  Many 
broken  bundles  of  fibers,  representing  the  thalamic  radiation,  are  seen  enter- 
ing the  capsule  upon  its  medial  surface. 

THE  CONNECTIONS  OF  THE  CORPUS  STRIATUM  AND  THALAMUS 

What  is  the  function  of  the  corpus  striatum,  and  what  connection  does  it 
have  with  other  parts  of  the  nervous  system?     These  questions,  to  which  no 


Caudate  nucleus 


Thalamus 


Parietal  stalk  of  thalamus 
Corticospinal  tract 

Insula 

Claustrum 

Putamen 

Globus  pallidus 

Ansa  pcditncularis 

Red  nucleus 

\lnsa  lenticularis 
'Substantia  nigra 
Hypothalamic  nucleus 

Fig.  196. — Diagram  of  the  connections  of  the  caudate  and  lenticular  nuclei. 

final  answer  can  as  yet  be  given,  have  recently  become  of  great  importance, 
because  of  the  frequency  with  which  degeneration  of  the  lentiform  nucleus  has 
been  found  at  autopsy  in  patients  who  have  shown  serious  disturbances  of  the 
motor  mechanism  (Wilson,  1912-1914).  It  seems  probable  that  the  corpus 
striatum  exerts  a  steadying  influence  upon  muscular  activity,  the  abolition  of 
which  results  in  tremor  during  voluntary  movement.  The  probable  connec- 
tions of  the  corpus  striatum  are  indicated  in  Fig.  196.  Strio petal  fibers  reach 
the  caudate  nucleus  from  the  anterior  and  medial  nuclei  of  the  thalamus  (Sachs, 
1909).  According  to  Cajal,  the  corpus  striatum  also  receives  collaterals  from 
the  corticospinal  tract.  Intemuncial  fibers  join  together  various  parts  of  the 
corpus  striatum.     The  majority  of  these  seem  to  run  from  the  caudate  nucleus 


THE    [NTERNAL   CONFIGURATION    OP    THE    CEREBRAL   BEMISPHERES  263 

to  the  putamen,  on  the  one  hand,  and  from  the  putamen  to  the  globus  pallidus 
on  the  other.  The  striofugal  fibers  arise,  for  the  most  pari  at  Least,  in  the  globus 
pallidus.  They  arc  collected  into  a  bundle  of  transvefselj  dire*  ted  fibers,  known 
as  the  ansa  lenticularis  (Fig.  188),  which  is  distributed  to  the  thalamus,  red 
nucleus,  hypothalamic  nucleus,  and  substantia  nigra,  other  fibers  belonging 
to  the  same  general  system  break  through  the  ventral  third  of  the  internal 
capsule  to  reach  the  thalamus  (Wilson,  1^14).  The  importance  of  the  connec- 
tion with  the  red  nucleus  is  obvious,  since  by  way  of  the  rubrospinal  and  rubro- 
reticular tracts  the  corpus  striatum  is  able  to  exert  its  influence  upon  the  pri- 
mary motor  neurons  of  the  brain  stem  and  spinal  cord.  The  fibers  to  the  sub- 
stantia nigra  have  already  been  mentioned  under  the  name  strio nigral  tract 
(p.  164).  The  impulses  which  travel  along  them  are,  in  all  probability,  re- 
layed through  the  substantia  nigra  to  krwer  lying  motor  centers,  although  the 
functions  and  connections  of  this  large  nuclear  mass  are  still  obscure. 

The  Thalamic  Radiation. — We  are  now  in  position  to  understand  the  course 
and  distribution  of  the  fascicles,  which  unite  the  thalamus  with  the  cerebral 
cortex  and  which  consist  of  both  thalamocortical  and  corticothalamic  fibers.  This 
thalamic  radiation  may  be  divided  into  four  parts:  the  frontal,  parietal,  occip- 
ital, and  ventral  stalks  of  the  thalamus,  which  will  now  be  traced  as  fasciculi, 
without  reference  to  the  direction  of  conduction  in  the  individual  fibers. 

The  ventral  stalk,  or  inferior  peduncle  of  the  thalamus,  streams  out  of  the 
rostral  portion  of  the  ventral  thalamic  surface  and  is  directed  lateralward  under 
cover  of  the  lentiform  nucleus.  Some  of  these  fibers  belong  to  the  ansa  lentic- 
ularis and  run  from  the  lentiform  nucleus  to  the  thalamus.  The  others,  form- 
ing a  bundle  known  as  the  ansa  pcduncularis,  runs  lateralward  ventral  to  the 
lentiform  nucleus  and  are  distributed  to  the  cortex  of  the  temporal  lobe  and 
insula  (Fig.  196). 

The  frontal  stalk,  or  peduncle  of  the  thalamus,  consists  of  fibers  which  run 
through  the  anterior  limb  of  the  internal  capsule  from  the  lateral  thalamic 
nucleus  to  the  cortex  of  the  frontal  lobe  (Fig.  193),  and  in  small  part  to  the  cau- 
date nucleus  also. 

The  parietal  stalk,  or  peduncle,  emerges  from  the  lateral  surface  of  the 
thalamus,  and  runs  through  the  posterior  limb  of  the  internal  capsule  in  close 
association  with  the  great  motor  tracts  (Figs.  193,  196).  It  connects  the  lateral 
nucleus  of  the  thalamus  with  the  cortex  of  the  parietal  and  posterior  part  of  the 
frontal  lobe. 

Many  of  these  fibers,  especially  those  terminating  in  the  posterior  central 


264  THE    NERVOUS    SYSTEM 

Lryrus.  are  afferent  fibers  of  the  third  order  mediating  sensations  of  touch,  heat, 
cold,  and  perhaps  also  pain  as  well  as  sensations  from  the  muscles,  joints,  and 
tendons  (Head.  1918).  These  sensory  fibers  are  located  behind  the  corticospinal 
tract  in  the  posterior  limb  of  the  internal  capsule.  According  to  Wilson  (1914) 
the  medullary  lamina'  of  the  lentiform  nucleus  do  not  contain  any  thalamocor- 
tical fibers. 

The  occipital  stalk,  or  peduncle,  is  also  known  as  the  optic  radiation  and  as 
the  radiatio  occipitothalamica.  Its  fibers  stream  out  of  the  pulvinar  and  lateral 
geniculate  body,  pass  through  the  retrolenticular  part  of  the  internal  capsule, 
and  run  in  a  curved  course  toward  the  occiput,  around  the  lateral  side  of  the 
rior  horn  of  the  lateral  ventricle  to  the  cortex  of  the  occipital  lobe,  and  es- 
pecially to  the  region  of  the  calcarine  fissure  (Figs.  190.  191).  It  also  contains 
some  fibers  arising  in  the  occipital  cortex  and  ending  in  the  superior  quadrigeminal 
body.  We  have  learned  that  it  forms  an  important  part  of  the  visual  path 
Fig.  162). 

Closely  associated  with  the  optic  radiation  in  the  retrolenticular  part  of  the 
internal  capsule  is  the  acoustic  radiation  (radiatio  thalamotemporalis).  This 
connects  the  medial  geniculate  body  with  the  anterior  transverse  temporal  gyrus 
and  the  adjacent  part  of  the  superior  temporal  gyrus,  and  mediates  auditory 
sensations.     It  should  be  included  as  a  part  of  the  thalamic  radiation. 


CHArTER  XVII 

THE  RHINENCEPHALON 

The  olfactory  portions  of  the  cerebral  hemisphere  may  all  be  grouped  to- 
gether under  the  name  rkinencephalon.  Phylogenetically  very  old.  this  part  of 
the  brain  varies  greatly  in  relative  importance  in  the  different  classes  of  verte- 
brates. The  central  connections  of  the  olfactory  nerves  form  all  or  almost  all  of 
the  cerebral  hemispheres  in  the  selachian  brain  (Fig.  13);  while  in  the  mammal 
the  non-olfactory  cortex  or  neopallium  has  become  the  dominant  part.  Even 
among  the  mammals  there  is  great  variation  in  the  importance  and  relative 
size  of  the  olfactory  apparatus.  The  rodents,  for  example,  depend  to  a  great 
extent  on  the  sense  of  smell  in  their  search  for  food,  and  possess  a  highly  developed 
rhinencephalon.  Such  mammals  are  classed  as  macrosmatic.  Man,  on  the 
other  hand,  belongs  in  this  respect  with  the  microsmatic  mammals,  because  in 
his  activities  the  sense  of  smell  has  ceased  to  play  a  very  important  part,  and 
his  olfactory  centers  have  undergone  retrogressive  changes.  The  carnivora  and 
ruminants  are  in  an  intermediate  group.  The  sheep's  brain  furnishes  a  good 
illustration  of  this  intermediate  type,  and  displays  much  more  clearly  than  the 
human  brain  the  various  parts  of  the  rhinencephalon  and  their  relation  to  each 
other. 

Parts  Seen  on  the  Basal  Surface  of  the  Brain. — A  comparison  of  the  basal 
surface  of  the  sheep's  brain  with  that  of  the  human  fetus  of  the  fifth  month  shows 
a  remarkable  similarity  in  the  parts  under  consideration  (Figs.  197.  198).  The 
olfactory  bulb,  which  is  the  olfactory  center  of  the  first  order,  is  oval  in  shape  and 
attached  to  the  hemisphere  rostral  to  the  anterior  perforated  substance.  It 
lies  between  the  orbital  surface  of  the  cerebral  hemisphere  and  the  cribriform 
plate  of  the  ethmoid  bone.  Through  the  openings  in  this  plate  numerous  fine 
filaments,  the  olfactory  nerves,  reach  the  bulb  from  the  olfactory  mucous  mem- 
brane. It  contains  a  cavity,  the  rhinoccele.  continuous  with  the  lateral  ventricle 
(Fig.  182).  In  the  adult  human  brain  the  cavity  is  obliterated  and  the  connec- 
tion between  bulb  and  hemisphere  is  drawn  out  into  the  long  olfactory  tract. 
This  is  lodged  in  the  olfactory  sulcus  on  the  orbital  surface  of  the  frontal  lobe 
and  in  transverse  section  presents  a  triangular  outline  (Fig.  172).     It  contains 

265 


266 


THE    NERVOUS    SYSTEM 


olfactory  fibers  of  the  second  order  connecting  the  bulb  with  the  secondary  ol- 
factory centers  in  the  hemisphere.  At  its  point  of  insertion  into  the  hemisphere 
the  olfactory  tract  forms  a  triangular  enlargement,  the  olfactory  trigone. 

From  the  point  of  insertion  of  the  olfactory  bulb  or  tract  a  band  of  gray 
matter,  the  medial  olfactory  gyrus,  can  be  seen  extending  toward  the  medial 
surface  of  the  hemisphere  (Figs.  159,  197,  198).  A  similar  gray  band,  the  lateral 
olfactory  gyrus,  runs  caudalward  on  the  basal  surface  of  the  sheep's  brain.    Along 


Longitudinal  fissure  of  ccrcbriu 
Optic  nerve 
Optic  chiasma^ 
Rhinal  fissure  J 

Insula— 
Lateral  fissure. 

Optic  tract . 

Infundibulum  - 

Mammillary  body  - 

Cerebral  pedunch 
Interpeduncular  fossa  and 
nucleus 

Trigeminal  nerve 
Abducens  nerve--- 

Acoustic{Veslibular  "-- 

nerve    ]  ~    , , 

[toe  lit  car  n. 

Glossopharyngeal  nerve 

Vagus  nerve'' 
Hypoglossal  nerve' 
Anterior  median  fissure 


Olfactory  bulb 

'  Medial  olfactory  gyrus 
Anterior  perforated  substance 
{■'Lateral  olfactory  stria 
Lateral  olfactory  gyrus 
--—Diagonal  band 

Amygdaloid  nucleus 

-  Pyriform  area 

-  Ilippocampal  gyrus 
■   Trochlear  nerve 

T,-'-Pons 

..-'  Abducens  nerve 

.--  Facial  nerve 

■tl'i^r; Trapezoid  body 

— Cerebellum 
Olive 
^'Chorioid  plexus 
*  Accessory  nerve 
^Tractus  lateralis  minor 


Fig.  197. — Ventral  view  of  the  sheep's  brain. 


its  lateral  border  it  is  separated  from  the  neopallium  by  the  rhinal  fissure;  while 
its  medial  border  contains  a  band  of  fibers,  the  stria  olfactoria  lateralis  (Fig.  197). 
The  same  gyrus  is  seen  in  the  brain  of  the  human  fetus,  but  here  it  is  directed 
outward  toward  the  insula  (Fig.  198).  In  the  adult  human  brain  these  olfactory 
convolutions  are  very  inconspicuous,  and  with  the  fibers  from  the  olfactory  tract 
which  accompany  them  are  usually  designated  as  the  medial  and  lateral  olfactory 
stria. 


Till:    RHINKNCKI'HALON 


267 


The  medial  olfactory  gyrus  and  stria  require  further  investigation.     It  has  been  gen- 
erally supposed  that  the  stria  is  formed  by  olfactory  fibers  of  the  second  and  third  order 

running  to  the  olfactory  centers  in  tin-  rostral  part  of  the  medial  surface  of  the  hemisphere. 
These  are  certainly  few  in  number  in  the  higher  mammals,  and  Cajal  (1(>11),  who  worked 
Chiefly  with  rodents,  has  been  unable  to  identify  any  SUCh  fibers  in  these  animals.  'The  sig- 
nificance of  the  medial  olfactory  gyrus  is  also  obscure.  According  to  Elliol  Smith  (1915), 
"the  rudiment  of  the  hippocampal  formation  that  develops  on  the  medial  surface  begins 
in  front  alongside  the  place  where  the  stalk  of  the  olfactory  peduncle  (which  becomes  the 
trigonum  olfactorium)  is  inserted;  it  passes  upward  to  the  superior  end  of  the  Lamina  tcrmi- 
nalis,  from  the  rest  of  which  it  is  separated  by  a  triangular  mass  of  gray  matter  called  the 
corpus  paraterminale"  I  Fig.  2(H)).  This  description,  as  well  as  the  figure  which  accompanies 
il,  suggests  a  close  relation  between  the  rostral  end  of  the  hippocampal  rudiment  and  what 
is  ordinarily  known  as  the  medial  olfactory  gyrus.  The  subdivision  of  the  olfactory  lobe 
into  anterior  and  posterior  portions  by  the  morphologically  unimportant  sulcus  parol 
factorius  posterior,  although  adopted  in  the  B.  N.  A.,  is  without  justification  and  leads  only 
to  confusion  (Elliot  Smith,  1907). 


Olfactory  bulb 


Lateral  olfactory  gyrus  (stria) 

Posterior  parolfactory  sulcus 

A  mygdaloid  nucleus 


Medial  olfactory  gyrus  (stria) 
Olfactory  tract 

Limen  insula 

Anterior  perforated  substance 

Hippocampal  gyrus 


Fig.  198. — Brain  of  a  human  fetus  of  22.5  cm.     Ventral  view.     (Retzius,  Jackson-Morris.) 

Between  the  olfactory  trigone  and  the  medial  olfactory  gyrus,  on  the  one 
hand,  and  the  optic  tract  on  the  other,  is  a  depressed  area  of  gray  matter  known 
as  the  anterior  perforated  substance,  through  the  openings  in  which  numerous 
small  arteries  reach  the  basal  ganglia  (Figs.  172,  197).  The  part  immediately 
rostral  to  the  optic  tract  forms  a  band  of  lighter  color,  known  as  the  diagonal 
gyrus  of  the  rhinencephalon  or  the  diagonal  band  of  Broca  (Fig.  197).  This 
can  be  followed  on  to  the  medial  surface  of  the  hemisphere,  where  it  is  continued 
as  the  paraterminal  body  or  subcallosal  gyrus  (Fig.  200).  Rostral  to  this  gyrus 
the  hippocampal  rudiment,  which  corresponds  in  part  to  the  parolfactory  area 
of  Broca,  extends  as  a  narrow  band  from  the  rostrum  of  the  corpus  callosum 
toward  the  medial  olfactory  gyrus.  In  those  mammals  which  possess  an  espe- 
cially rich  innervation  of  the  nose  and  mouth,  the  region  of  the  anterior  per- 
forated space  is  marked  by  a  swelling,  sometimes  of  considerable  size,  called 


268 


THE    NERVOUS    SYSTEM 


the  tubcrculum  olfactorium.  According  to  Retzius,  a  small  oval  mass  is  present 
in  the  anterior  perforated  substance  of  man  immediately  adjacent  to  the  ol- 
factory  trigone,  which  represents  this  tubercle. 

Olfactory  bulb 

Anterior  commissure 
interior  perforated  substance 
■A  mygdaloid  nucleus 
Pyriform  area 


Fig.   199. — Ventral   view  of  a   sheep's  brain,   pyriform  area   shaded  and  anterior  commissure 

exposed. 

The  Pyriform  Area. — The  lateral  olfactory  gyrus  is  continuous  at  its  caudal 
extremity  with  the  hippocampal  gyrus  (Figs.  197.  198).  and  the  two  together 
form  the  pyriform  area  or  lobe  (Tig.  199).  In  the  adult  human  brain  it  is  more 
difficult  to  demonstrate  the  continuity  of  these  parts.     As  the  temporal  lobe  is 


Hippocampal  rudiment  -v 
Corpus  callosum 
Septum  pellucid um  - 
Fornix 
Anterior  commissure . 


Parat-rminal  body  ^ 
Hippocampal  rudiment 
Olfactory  trigone 
Olfaxtory  tract 
Olfactory  bulbK 


Intermediate  olfactory  stria' 
Lateral  olfactory  gyrus  and  stria' 


Anterior  perforated  substance 

Limen  insula • 


.Hippocampus  (gyri 
Andrea;  Rctzii) 

)  —  Fascia  dentata 

-  Fimbria  of  hippocampus 

Hippocampus  (proper) 
'"--  Hippocampus 
~  Hippocampal  gyrus 
'*■  Cauda  fascia;  dentata 
Uncus 


Diagonal  band 
Fig.  200. — Diagram  of  the  rhinencephalon. 


thrust  rostrally  and  the  insula  becomes  depressed,  the  pyriform  area  is  bent 
on  itself  like  a  V  (Fig.  198).  The  knee-like  bend  forms  the  limen  insula,  and 
with  the  rest  of  the  insula  becomes  buried  at  the  bottom  of  the  lateral  fissure. 
The  continuity  of  the  pyriform  area  is  not  interrupted  in  the  adult,  though  part 


THE    RHINEN<  I  I'll  \l,u\ 


of  it  is  hidden  from  view.  It  includes  the  lateral  olfactory  stria  and  the  cortex 
subjacent  to  it  (or  lateral  olfactory  gyrus),  the  limen  insula-,  the  uncus,  and  at 
least  a  part  of  the  kippocampal  gyrus  (Figs.  K>().  172.  200).  It  i-  not  easy  to 
determine  just  how  much  of  the  human  hippocampa]  gyrus  should  be  included. 
Cajal  (1911)  apparently  includes  the  entire  gyrus,  while  Elliol  Smith  (1915) 
limits  it  to  the  part  of  the  gyrus  dorsal  to  the  rhinal  fissure.  In  Fig.  200  we 
have  followed  the  outlines  of  the  hippocampa]  region  as  given  by  Brodmann 
(1909). 

The  Hippocampus.-  An  olfactory  center  of  still  higher  order  is  represented 
by  the  hippocampus,  which  was  seen  in  connection  with  the  study  of  the  lateral 


Inferior  horn  of  lateral  ventr'n le 


Hippocampus 


Collateral  eminence. 


Tapelitm 
C  'ollateral  trigone 


Posterior  horn  of  lateral  ventricle 


Hippocampal  digitations 


■  I'm  us 


Dentate  fascia  of  hippocampus 
Hippocampal  gyrus 
Hippocampa!  fissure 

'imbria  of  hippocampus 


Bulb  of  posterior  horn 
Calcarine  fissure 


Calcar  avis 


Fig.  201. — Part  of  temporal  lobe  of  human  brain  showing  inferior  horn  of  lateral  ventricle  and  the 
hippocampus.     Dorsal  view.     (Sobotta-McMurrich.) 


ventricle.  If  we  turn  again  to  the  floor  of  the  inferior  horn  of  the  lateral  ven- 
tricle we  shall  see  a  long  curved  elevation  projecting  into  the  cavity  (Figs.  181, 
201).  This  is  the  hippocampus  and  is  formed  by  highly  specialized  cortex 
which  has  been  rolled  into  the  ventricle  along  the  line  of  the  hippocampal  fissure 
(Figs.  204,  209).  It  is  covered  on  its  ventricular  surface  by  a  thin  coating  of 
white  matter,  called  the  alveus,  which  is  continuous  along  its  medial  edge  with 
a  band  of  fibers  known  as  the  fimbria  of  the  hippocampus.  This,  in  turn,  is 
continuous  with  the  fornix  (Fig.  201).  In  Figs.  201  and  204  there  may  be  seen, 
along  the  border  of  the  fimbria,  a  narrow  serrated  band  of  gray  matter,  the 
fascia  dentata,  which  lies  upon  the  medial  side  of  the  hippocampus.  It  is  sepa- 
rated from  the  hippocampal  gyrus  by  a  shallow  groove,  called  the  hippocampal 


270  THE    NERVOUS    SYSTEM 

fissure,  that  marks  the  line  along  which  the  hippocampus  has  been  rolled  into 
the  ventricle. 

The  hippocampus  and  fascia  dentata  belong  to  the  archipallium.  In  the 
marsupials  and  monotremes  this  extends  dorsally  on  the  medial  surface  of  the 
hemisphere  in  a  curve,  which  suggests  that  of  the  corpus  callosum  (Fig.  202). 
In  the  higher  mammals  the  presence  of  a  massive  corpus  callosum  seems  to 
inhibit  the  development  of  the  adjacent  part  of  the  hippocampal  formation, 
which  remains  as  the  vestigial  indusium  griseum,  or  supracallosal  gyrus.  This 
hippocampal  rudiment  is  a  thin  layer  of  gray  matter  on  the  dorsal  surface  of  the 
corpus  callosum,  within  which  are  found  delicate  strands  of  longitudinal  fibers. 
Two  of  these  strands,  placed  close  together  on  either  side  of  the  median  plane, 

Cerebral  cortex 
; — ^^     .Hippocampal  fissure 

yr  •  x.      Hippocampus  and  fascia 

/  /  /(     dentata 

I  j<S^  ^"N.  y  \*  Chorioid  fissure 

S — "NA  ^^1$$£$/^  ^^^^yy^y^^m!^  \ 

f  \  j£r       I        \^wv^%^7/^^^h™  ■»«•••(■■•  Thalamus 


Olfactory  bulb  /^^^^^^  S  ~~~~~j*C    „     ., 

/       ^^"^\_^^     ^yC  /    "  Pyriform  area 

Tubercukim  olfaciorium  n^  "^^y 

^-      ^^       "  Rhinal  fissure 

Fig.  202. — Median  view  of  the  cerebral  hemisphere  of  a  monotreme  Ornithorhynchus.     (Elliot 

Smith.) 

are  more  conspicuous  than  the  others,  and  are  known  as  the  medial  longitudinal 
strice.  On  either  side,  where  the  supracallosal  gyrus  bounds  the  sulcus  of  the 
corpus  callosum,  there  is  a  less  distinct  strand,  the  lateral  longitudinal  stria 
(Figs.  174,  175).  The  hippocampal  rudiment  can  be  traced  upon  the  medial 
surface  of  the  hemisphere  from  the  region  of  the  medial  olfactory  gyrus  (or  stria) 
toward  the  rostrum  of  the  corpus  callosum,  then  around  the  dorsal  surface  of 
that  great  commissure  to  the  splenium,  behind  which  it  becomes  continuous 
with  the  hippocampus  proper,  where  this  comes  to  the  surface  in  the  angle 
between  the  fascia  dentata  and  the  hippocampal  gyrus  (Fig.  200 — Elliot  Smith, 
1915). 

The  Fornix. — Within  the  hippocampus  fibers  arise  which  run  through  the 
white  coat  on  its  ventricular  surface,  known  as  the  alveus,  into  the  fimbria.     This 


Till       KIIIM   \(   I   I'llAl.uX 


271 


is  a  thin  band  of  fibers,  running  along  the  medial  surface  of  the  hippocampus 
and  joining  with  the  alveus  to  form  the  floor  of  the  inferior  horn  of  the  lateral 
ventricle  (Figs.  201,  204,  209).    The  fimbria  increases  in  volume  as  it  is  traced 

toward  the  splenium  of  the  corpus  callosum,  to  the  under  surface  of  which  it 
becomes  applied,  where,  together  with  its  fellow  of  the  oppo>ite  side,  it  forms 
the  fornix. 

The  fornix,  which  is  represented  diagrammatically  in  Fig.  203.  is  an  arched 
filter  tract,  consisting  of  two  symmetric  lateral  halves,  whieh  are  separate  at 
either  extremity,  but  joined  together  beneath  the  corpus  callosum.  This 
medially  placed  portion  is  known  as  the  body  of  the  fornix.  From  its  caudal 
extremity  the  fimbria  diverge,  and  one  of  them  runs  along  the  medial  aspect  of 
each  hippocampus.     In  man  the  hippocampus  does  not  reach  the  under  surface 


Column  of  fornix 


Body  of  fornix 


Hippocampal  commissure 
—  Cms  of  fornix 


hippocampus 


Fig.  203. — Diagram  of  the  fornix. 


of  the  corpus  callosum,  and  the  part  of  the  fimbria  which  joins  the  body  of  the 
fornix,  being  unaccompanied  by  hippocampus,  is  known  as  the  cms  fornicis. 
Rostrally  the  fornix  is  continued  as  two  arched  pillars,  the  columna  fornicis, 
to  the  mammillary  bodies. 

The  body  of  the  fornix  is  triangular,  with  its  apex  directed  rostrally.  It  con- 
sists in  large  part  of  two  longitudinal  bundles  of  fibers,  representing  the  con- 
tinuation of  the  fimbria?,  widely  separated  at  the  base  of  the  triangle,  but  closely 
approximated  at  the  apex,  whence  they  are  continued  as  the  columns  fornicis. 
At  the  point  where  these  longitudinal  bundles  diverge  toward  the  base  of  the 
triangle  they  are  united  by  transverse  fibers  which  join  together  the  two  hippo- 
campi by  way  of  the  fimbria?.  These  fibers  constitute  the  hippocampal  com- 
missure. This  part  of  the  fornix,  because  of  its  resemblance  to  a  harp,  was 
formerly  known  as  the  psalterium  (Fig.   184).     The  hippocampal  commissure 


272 


THE    NERVOUS    SYSTEM 


is  not  very  evident  in  the  human  brain,  but  can  be  easily  dissected  out  in  the 
sheep  (Fig.  204). 

The  coin  nunc  J  or  n  iris  are  round  fascicles  which  can  be  traced  ventrally  in 
an  arched  course  to  the  mammillary  bodies  (Figs.  203-205).  They  are  placed 
on  either  side  of  the  median  plane.  Each  consists  of  an  initial  free  portion, 
which  forms  the  rostral  boundary  of  the  interventricular  foramen,  and  a  cov- 
ered part,  which  runs  through  the  gray  matter  in  the  lateral  wall  of  the  third 
ventricle  to  reach  the  mammillary  body  (Figs.  204,  205). 

The  relations  of  the  fornix  are  well  shown  in  Figs.  155,  200,  and  205.  The 
body  of  the  fornix  intervenes  between  the  corpus  callosum,  septum  pellucidum, 


Body  of  corpus  callosum 

Lateral  ventricle         \ 

Genu  of  corpus  callosum 


Body  of  fornix 

'         Hippocampal  commissure 
,'  Thalamus 


Splenium  of  corpus  callosum 
_  Lateral  ventricle 
r«  Chorioid  fissure 
~  Hippocampus 

-  Fimbria  of  hippo- 
campus 
-/---  Hippocampal 

fissure 
--■- —  Hippocampal  gyrus 

Dentate  fascia 
Mammittothalamic  tract 
Mammillary  body 
Infundibulum 
Fig.  204. — Dissection  of  the  cerebral  hemisphere  of  the  sheep  to  show  the  fornix  and  hippocampus. 


Anterior  commissure 


Lamina  lerminaHs     I 
Optic  chiasma 

Column  of  forni.\\ 


Median  view. 


and  cavity  of  the  lateral  ventricle  on  the  one  hand,  and  the  transverse  fissure  of 
the  cerebrum  and  the  thalamus  on  the  other.  The  fimbria  and  body  of  the  for- 
nix form  one  boundary  of  the  chorioid  fissure.  This  fissure,  which  is  shown  but 
not  labeled  in  Fig.  205,  represents  the  line  along  which  the  chorioid  plexus  is 
invaginated  into  the  lateral  ventricle.  When  this  plexus  has  been  torn  out, 
the  fissure  communicates  with  the  interventricular  foramen. 

The  septum  pellucidum  is  the  thin  wall  which  separates  the  two  lateral  ven- 
tricles and  fills  in  the  triangular  interval  between  the  fornix  and  the  corpus 
callosum  (Fig.  205).  It  consists  of  two  thin  vertical  lamina?  separated  by  a 
cleft-like  interval,  the  cavity  of  the  septum  pellucidum  (Fig.  177).     Each  lamina 


TIIK    KlIINK.\(i:i'HAI.().\ 


273 


forms  part  of  tin-  medial  wall  of  the  corresponding  hemisphere  <li,^r.  182);  and 
tin-  cavity,  although  sometimes  called  tin-   fifth  ventricle,  develops  as  a  cleft 

within  the  lamina  iciniina lis  and,  therefore,  bears  no  relation  to  the  true  brain 
ventricles,  which  are  expansions  of  the  original  lumen  of  tin  mural  tube  (Fig. 
Wo). 

The  anterior  commissure,  like  the  hippocampal  commissure,  belongs  to  the 
rhinencephalon.  It  is  a  rounded  fascicle  which  crosses  tin-  median  plane  in  the 
dorsal  part  of  the  lamina  terminalis  just  rostral  to  the  columnae  fornicus  (Fig. 
205).      In  a  frontal  section  of  the  brain,  like  that  represented  in  Fig.  1X7,  it  can 


Splenium  of  corpus  callosum 
Sulcus  cinguli 

Parieto-occipiial  f 


Cumus 
Calcarine  \ 
Jissure        . 


Body  of  fornix 


Body  of  corpus  callosum 

Free  portion  of  col.  of  fornix 
Septum  piiha  \ilum 
Inter  vent,  foramen 
.1  nlerior  commiss. 

Genu  of 

corpus 
callosum 


Occipita- 
lobe 

Cms  of  fornix 

Thalamus  m^ 

Fimbria  of  hippocampus 
Dentate  fascia  of  liippocampu 

Mammillothalamic  tract 


Uncus 


Olfactory 
bulb 
Olfactory  trad 
Rostrum  of  corpus  col. 
\     \    Rostral  lamina 
'       Optic  nerve 

''Covered  portion  of  column  of 
Mammillary  body  fornix 


Fig.   205. — Dissection  of  the   human   cerebral   hemisphere  to  show  the  fornix.     Median   view. 

(Sobotta-McMurrich.) 


be  traced  lateralward  through  the  most  ventral  part  of  the  lentiform  nucleus. 
It  consists  of  two  parts  (Fig.  206).  Of  these,  the  more  rostral  is  shaped  like  a 
horseshoe  and  joins  together  the  two  olfactory  bulbs.  This  part  can  be  readily- 
dissected  out  in  the  sheep's  brain  (Fig.  199),  but  is  poorly  developed  in  man.  The 
remaining  portion,  and  in  man  the  chief  component,  joins  the  pyriform  areas 
of  the  two  hemispheres  together  (Cajal,  1911). 

We  are  now  sufficiently  acquainted  with  the  anatomy  of  the  rhinencephalon 
to  undertake  a  study  of  the  structure  and  connections  of  its  various  parts. 
Because  of  the  wealth  of  detail  which  this  subject  offers  we  must  confine  our  at- 


274 


TIIK    NKRVOUS    SYSTEM 


tention  to  the  more  important  facts.  Cajal  (1911)  has  carried  out  extensive 
investigations  concerning  the  structure  and  connections  of  the  olfactory  parts 
of  the  brain  both  in  man  and  the  smaller  macrosmatic  mammals,  especially  the 
mouse.  His  results,  which  differ  in  many  respects  from  the  ideas  previously 
current,  have  been  brought  together  in  his  "Histologic  du  Systeme  Nerveux," 
Vol.  II,  pp.  646-823.     The  account  which  follows  is  largely  based  on  his  work. 


Fig.  206. — Horizontal  section  of  the  rostral  portion  of  the  cerebral  hemispheres  of  a  mouse  to 
show  the  anterior  commissure.  Golgi  method.  A,  anterior  and  B,  posterior  portions  of  anterior 
commissure;  G,  anterior  column  of  the  fornix.      (Cajal.) 

Structure  and  Connections  of  the  Olfactory  Bulb. — In  the  olfactory  portion 
of  the  nasal  mucous  membrane  there  are  located  bipolar  sensory  cells,  each  with  a 
thick  peripheral  process,  the  ciliated  extremity  of  which  reaches  the  surface  of 
the  epithelium.  These  are  the  olfactory  neurons  of  the  first  order,  and  their 
slender  central  processes  are  the  unmyelinated  axons  which  constitute  the  olfac- 
tory nerves.     These  fibers  are  gathered  into  numerous  small  bundles,  the  fila- 


THE    RHINI   \(   I  I'll  VI.O.N 


275 


ments  of  the  olfactory  >i<r:<\  which  pass  through  the  cribriform  plate  of  the  eth- 
moid bone  ami  immediately  enter  the  olfactory  bulb  (Fig.  207).  Hen-  they 
form  a  feltwork  of  interlacing  fibers  over  that  surface  of  the  hull)  which  is  in 
contact  with  the  cribriform  plate. 

'The  olfactory  bulb  of  man  is  solid,  and  the  original  cavity  is  represented  by  a 
central  gray  mass  of  neuroglia.     'This  is  surrounded  by  a  deep  layer  of  myelinated 


Fig.  207. — Diagram  showing  the  direction  of  conduction  in  the  olfactory  nerve  bulb  and  tract: 
A,  lateral  olfactory  stria;  B,  anterior  portion  of  the  anterior  commissure;  C,  bipolar  cells  of  the 
olfactory  mucous  membrane.     (Cajal.) 


nerve-fibers  passing  to  and  from  the  olfactory  tract.  Superficial  to  this  are  several 
layers  of  gray  matter  of  very  characteristic  structure,  and  this,  in  turn,  is  covered 
with  the  superficial  layer  of  unmyelinated  fibers  from  the  olfactory  nerve  fila- 
ments. Within  the  gray  matter  of  the  bulb  are  found  three  types  of  neurons, 
the  mitral,  tufted,  and  granule  cells.     The  large  mitral  cells  are  the  most  char- 


276 


THE    NERVOUS    SYSTEM 


acteristic  element;  and  their  perikarya  are  closely  grouped  together,  forming 
a  well-defined  layer  (Fig.  208,  C).  The  tufted  cells  are  smaller  and  more  super- 
ficially placed  (Fig.  208,  B).  The  larger  dendrites  from  both  these  types  of 
neurons  are  directed  toward  the  superficial  fiber  layer.     Each  of  these  dendrites 


Fig.  208. — Section  of  the  olfactory  bulb  of  a  kitten.  Golgi  method.  A,  Layer  of  glomeruli; 
B,  external  plexiform  layer;  C,  layer  of  mitral  cells;  D,  internal  plexiform  layer;  E,  layer  of  granules 
and  white  substance;  /,  J,  granule  cells;  a,  b,  glomeruli,  showing  the  terminations  of  the  olfactory 
nerve-fibers;  c,  glomerulus,  showing  the  terminal  arborization  of  a  dendite  of  a  mitral  cell;  d, 
tufted  cells;  e,  mitral  cell;  h,  recurrent  collateral  from  an  axon  of  a  mitral  cell.     (Cajal.) 

breaks  up  into  many  branches,  which  form  a  compact  rounded  bushy  terminal. 
The  terminal  ramifications  of  olfactory  nerve-fibers  interlace  with  these  dendritic 
branches,  and  the  two  together  form  a  circumscribed,  more  or  less  spheric  ol- 
factory glomerulus  (Fig.  208,  A).     These  relations  were  demonstrated  by  Cajal 


THK    RHINENCEPHALON  277 

in  1800,  and  possess  considerable  theoretic  and  historic  interest.  Since  in  these 
glomeruli  the  olfactory  nerve-fibers  come  into  contact  with  only  the  dendritic 
ramifications  of  the  mitral  and  tufted  cells,  it  is  evidenl  thai  these  dendrites 
must  take  up  and  transmit  the  olfactory  impulses.  That  is  to  say,  these  glomer- 
uli furnished  positive  proof  that  the  dendrites  are  not,  as  had  been  thoughl  by 
many  investigators,  merely  root-like  branches  which  serve  for  the  nutrition  of 
the  cell.  The  mitral  cells  are  larger  than  the  tufted  cells  and  their  axons  are 
thicker.  These  coarse  axons  are  directed  for  the  most  part  into  the  lateral  ol- 
factory stria;  while  the  finer  axons  of  the  tufted  cells  pass  through  the  (interior 
commissure  to  the  opposite  olfactory  bulb  (Fig.  207).  The  axons  of  the  deeply 
placed  granule  cells  arc  relatively  short  and  are  directed  toward  the  surface  of 
the  bulb. 

The  olfactory  tract  consists  of  fibers  passing  to  and  from  the  olfactory  bulb. 
Through  it  each  bulb  receives  fibers  from  the  other  by  way  of  the  anterior  com- 
missure as  well  as  from  the  hippocampal  cortex.  The  fibers  leaving  the  olfac- 
tory bulb  are  the  axons  of  the  mitral  and  tufted  cells.  By  far  the  greater  number 
of  the  axons  of  the  mitral  cells  are  continued  into  the  lateral  olfactory  stria.  A 
much  smaller  number  terminates  in  the  olfactory  trigone  and  in  the  tuberculum 
olfactorium  within  the  anterior  perforated  substance.  Other  fibers  are  said  to 
pass  by  way  of  the  medial  olfactory  stria  to  the  parolfactory  area  of  Broca,  to 
the  subcallosal  gyrus,  and  to  the  septum  pellucidium,  but  this  is  open  to  ques- 
tion. The  fibers  of  the  lateral  olfactory  stria  run  upon  the  surface  of  the  lateral 
olfactory  gyrus,  also  knowm  as  the  frontal  olfactory  cortex,  to  which  they  give 
off  collaterals  (Fig.  207).  The  terminal  fibers  reach  the  uncus  and  part  of  the 
hippocampal  gyrus.  The  chief  olfactory  centers  of  the  second  order  are,  there- 
fore, found  in  the  pyriform  area. 

According  to  Cajal  (1911),  the  hippocampal  gyrus  may  be  subdivided  in  man,  as  in  the 
mammals,  into  five  areas:  (1)  the  external  region  near  the  rhinal  fissure;  (2)  the  principal 
olfactory  region,  the  most  salient  part  of  the  convolution;  (3)  the  presubiculum,  a  transitional 
area  between  2  and  4;  (4)  the  subiculum,  near  the  hippocampal  fissure,  and  (5)  the  caudal 
olfactory  region,  including  the  caudal  part  of  the  hippocampal  gyrus.  Of  these  five  regions, 
Cajal  finds  fibers  from  the  lateral  olfactory  stria  going  to  the  second  or  principal  olfactory 
region  only.  The  presubiculum  and  subiculum  and  the  caudal  olfactory  region  represent 
olfactory  association  centers.  The  subiculum  is  characterized  by  the  presence  of  a  thick 
layer  of  myelinated  fibers  upon  its  surface. 

The  hippocampus,  which  constitutes  an  olfactory  center  of  a  still  higher 
order,  is  directly  continuous  with  the  portion  of  the  hippocampal  gyrus  known 
as  the  subiculum  (Fig.  209),  and  is  formed  by  a  primitive  portion  of  the  cortex 


278 


THE    NERVOUS    SYSTEM 


that  has  been  rolled  into  the  ventricle  along  the  line  of  the  hippocampal  fissure. 
Upon  its  ventricular  surface  it  is  covered  by  a  thin  layer  of  white  matter,  known 
as  the  alveus,  through  which  the  fibers  arising  in  the  hippocampus  reach  the 
fimbria  and  the  fornix.  Beginning  at  the  line  of  separation  from  the  fascia 
dentata,  we  may  enumerate  the  constituent  layers  of  the  hippocampus  as  fol- 
lows: the  molecular  layer,  the  layer  of  pyramidal  cells,  and  the  layer  of  poly- 
morphic cells  (Figs.  209,  210). 

The  molecular  layer  contains  a  superficial  stratum  of  tangential  fibers  derived 
from  the  corresponding  layer  of  the  subiculum  and  from  bundles  of  fibers  that 


Fig.  209. — Cross-section  of  the  hippocampus  and  hippocampal  gyrus  of  man.      (Edinger.) 

perforate  the  cortex  of  the  subiculum  (Fig.  210).  More  deeply  placed  is  another 
fiber  layer,  containing  collaterals  from  the  pyramidal  cells  as  well  as  collateral  and 
terminal  fibers  from  the  alveus,  and  known  as  the  stratum  lacunosum.  The 
molecular  stratum  in  the  hippocampus  resembles  that  in  other  parts  of  the  cortex 
in  containing  the  terminal  branches  of  the  apical  dendrites  from  the  pyramidal 
cells,  and  a  few  nerve-cells  which  for  the  most  part  belong  to  Golgi's  Type  II. 
The  Layer  of  Pyramidal  Cells.— The  pyramidal  cells  are  all  of  medium  size 
and  their  fusiform  bodies  are  rather  closely  packed  together,  forming  a  well- 


THE    RHINKNCKl'IIAI.ON 


79 


defined  zone,  the  stratum  lucidum.  Their  apical  dendrites  are  dire*  led  toward 
the  molecular  layer  and  form  the  chief  constituent  of  the  stratum  radialum. 
The  axons  of  these  cells,  after  giving  off  collaterals,  enter  the  alveus. 

The  layer  of  polymorphic  cells,  also  known  as  the  stratum  oriens,  contains 
cells  of  Martinotti,  that  send  their  axons  into  the  molecular  layer,  and  :-,till  other 
cells  the  axons  of  which  enter  the  alveus. 

The  alveus  is  a  thin  white  stratum  which  separates  the  preceding  layer  from 
the  ventricle.     It  is  continuous,  on  the  one  hand,  with  the  white  center  of  the 


Alveus 
Layer  of  polymorphic  cells 

Laver  of  pyramidal 

cells, 

Stratum  lucidum\ 

Stratum  radia- 

tum 


.  Molecular  layer 
i  Stratum  lacunosum 
Tangential  fibers 


Lateral  ventricle 


Fimbria 


Fascia  denlata 


Subiculum 

'Granule  layer 

■  Layer  of  polymorphic  cells 

Fig.  210. — Diagram  of  the  structure  and  connections  of  the  hippocampus.  The  arrows 
show  the  direction  of  conduction:  A,  molecular  layer,  and  B,  pyramidal  cell  layer  of  the  subic- 
ulum; F,  hippocampal  fissure.      (Cajal.) 

hippocampal  gyrus,  and  on  the  other  with  the  fimbria.  Through  it  the  efferent 
fibers  of  the  hippocampus  enter  the  fimbria  and  fornix.  The  fibers  of  the  hippo- 
campal commissure  are  also  carried  in  the  fimbria  and  enter  the  hippocampus 
through  the  alveus. 

The  fascia  dentata  also  belongs  to  the  archipallium  and  is  closely  related  to 
the  hippocampus,  which  it  resembles  somewhat  in  the  structure  of  its  three 
strata:  the  molecular  layer,  granule  layer,  and  layer  of  polymorphic  cells  (Fig. 
210).  The  granules  may  be  regarded  as  modified  pyramidal  cells  of  small  size, 
ovoid  or  fusiform  in  shape.  Each  possesses  instead  of  a  single  apical  dendrite 
two  or  three  branching  processes  which  extend  into  the  molecular  layer.     The 


2  So  THE   NERVOUS    SYSTEM 

axons  are  directed  into  the  layer  of  pyramidal  cells  of  the  hippocampus.  Orig- 
inally this  layer  of  pyramidal  cells  was  continuous  with  the  granule  layer  of 
the  fas(  i;i  dentata,  but  in  all  the  higher  mammals  a  break  in  this  cellular  stratum 
has  occurred  at  the  point  of  transition  between  the  two  divisions  of  the  archi- 
pullium. 

THE  OLFACTORY  PATHWAYS 

Impulses  reach  the  glomeruli  of  the  olfactory  bulb  along  the  fibers  of  the 
olfactory  nerve  and  are  here  transferred  to  the  dendrites  of  the  mitral  cells. 
Axons  arising  from  these  cells  and  running  in  the  lateral  olfactory  stria  transmit 
the  impulses  to  the  pyriform  area  (Fig.  207),  whence  they  are  conveyed  to  the 
hippocampus  and  fascia  dentata  by  fibers  entering  the  molecular  layer  in  both 
of  these  parts  of  the  hippocampal  formation  (Fig.  210). 

According  to  Cajal.  the  fibers  of  the  lateral  olfactory  stria  terminate  in  the  principal 
olfactory  region  of  the  hippocampal  gyrus,  and  there  are  present  within  the  cortex  of  the 
pyriform  area  sagittal  association  fibers  which  unite  the  principal  olfactory  region  with  the 
caudal  olfactory  region  of  the  hippocampal  gyrus.  From  this  latter  region  fibers  reach  the 
hippocampus  and  fascia  dentata.  These  are  relatively  thick  fibers  which  are  found  at  first 
in  the  angle  of  the  subiculum  and  can  be  traced  through  all  the  layers  of  that  center  into 
the  molecular  layer  of  the  hippocampus  and  fascia  dentata  (Fig.  210,  B).  Within  the  molec- 
ular layer  the  impulses  are  transferred  from  these  fibers  to  the  dendrites  of  the  pyramidal 
and  granule  cells.  It  was  formerly  supposed  that  fibers  from  the  trigonum  olfactorium, 
substantia  perforata  anterior,  and  septum  pellucidum  reached  the  hippocampus  through 
the  striae  longitudinales  and  the  fornix,  and  served  as  the  chief  conductors  of  afferent  im- 
pulses toward  the  hippocampus.  But  according  to  Cajal,  "The  hippocampus  does  not  receive 
olfactory  impulses  from  the  frontal  region  of  the  brain,  nor  through  the  intermediation  of  the 
septum  pellucidum." 

The  efferent  fibers  from  the  hippocampus  represent  the  axons  of  the  pyra- 
midal cells.  These  penetrate  the  stratum  oriens  and  enter  the  alveus  (Fig. 
210).  Thence  they  are  continued  into  the  fimbria  and  fornix.  They  include 
both  commissural  and  projection  fibers.  The  commissural  fibers  serve  to  unite 
the  two  hippocampi  and  run  through  the  hippocampal  commissure  as  the  trans- 
verse fibers  of  the  psalterium.  The  projection  fibers  are  continued  rostrally; 
and  in  their  course  through  the  body  of  the  fornix  they  form  on  either  side  of 
the  median  plane  a  longitudinal  bundle,  which  is  continued  into  the  columna 
fornicis  (Fig.  203).  The  latter  bends  caudally  into  the  hypothalamic  region, 
giving  off  fibers  to  the  tuber  cinercum  and  the  mammUlary  body.  The  remaining 
fibers  of  the  columna  fornicis  undergo  a  decussation  just  behind  the  mamillary 
body  and  are  continued  in  the  reticular  formation  of  the  brain  stem  as  far,  at 
least,  as  the  pons.     It  will  be  obvious  that  the  fornix  is  the  efferent  projection 


THE    klllNENl  Kl'IIAI.ON 


28l 


tract  of  the  archipallium  and  servo  to  convey  impulses  from  the  hippocampus 
to  the  hypothalamus  and  reticular  formation  of  the  brain  stem.  Through  the 
mammillary  bodies  olfactory  impulses  are  relaxed  along  the  inanimillothalamic 

tract  to  the  anterior  nucleus  of  the  thalamus,  and  along  the  mammillotegmental 
bundle  to  the  tegmentum  of  the  pons  and  medulla  oblongata  (Fig.  211,/,  g). 

The  frontal  olfactory  projection  tract  takes  origin  from  the  gray  matter  of 
the  olfactory  peduncle  or  trigonum  olfactorium  and  the  gyrus  olfactorius  later- 


Fig.  211. — Diagram  of  the  afferent  and  efferent  paths  of  the  mammillary  body,  habenular 
ganglion,  and  interpeduncular  ganglion:  A,  Medial  nucleus  of  the  mammillary  body;  B,  C, 
anterior  nucleus  of  the  thalamus;  D,  habenular  ganglion;  E,  interpeduncular  ganglion;  F,  dorsal 
tegmental  nucleus;  J,  optic  chiasma;  T,  tuber  cinereum;  P,  pons;  a,  cerebral  aqueduct;  b,  habenular 
commissure;  c,  posterior  commissure;  d,  fasciculus  retroflexus  of  Meynert;  e,  peduncle  of  the  mam- 
millary body;/,  fasciculus  mamillothalamicus;  .?,  tegmental  tract  of  Gudden;  /;,  frontal  olfactory 
projection  tract;  i,  stria  medullaris  thalami.  The  arrows  indicate  the  direction  of  conduction. 
(Cajal.) 


alis.  It  traverses  the  subthalamic  region  to  reach  the  pons  and  medulla  oblon^ 
gata.  A  bundle  of  fibers,  consisting  in  part  of  collaterals,  is  given  off  from  it, 
to  enter  the  stria  medullaris  thalami,  which  we  have  already  traced  to  the  habe- 
nular ganglion  (Fig.  211,  //,  i). 

The  stria  terminalis  is  a  delicate  fascicle  of  nerve-fibers  which  lies  in  the  sulcus  between 
the  thalamus  and  caudate  nucleus  (Figs.  155,  177),  and  accompanies  the  tail  of  the  latter  in 


282  THE    NERVOUS    SYSTEM 

the  roof  of  the  inferior  horn  of  the  lateral  ventricle.  According  to  Cajal  (1911),  it  contains 
both  commissural  and  projection  fibers,  the  majority  of  which  take  origin  from  the  olfactory 
cortex  of  the  hippocampal  gyrus.  A  smaller  number  may  arise  in  the  amygdaloid  nucleus. 
After  following  the  curved  course  of  the  caudate  nucleus,  it  bends  ventrad  toward  the 
anterior  commissure.  Some  of  the  fibers  cross  in  the  anterior  commissure  and  end  in  the 
olfactory  cortex  of  the  opposite  temporal  lobe  and  in  the  septum  pellucidum.  The  majority 
of  the  fibers,  however,  enter  the  mesencephalon  and  apparently  end  in  the  interstitial  nucleus. 

The  striae  longitudinales.  fornix  longus.  and  the  fiber  tracts  found  in  the 
subcallosal  cortex  and  septum  pellucidum  have  apparently  been  subject  to 
much  misinterpretation;  but  the  subject  is  too  extensive  to  be  considered  here. 
(See  Cajal,  Histologic  du  Systeme  Xerveux.  Vol.  II.  pp.  783-823.) 

The  anterior  perforated  substance,  or  at  least  its  more  rostral  part,  which 
corresponds  to  the  tuberculum  olfactorium  of  macrosmatic  mammals,  receives 
besides  fibers  from  the  olfactory  tract  other  afferent  fibers  which,  according  to 
Edinger  (1911),  come  from  the  pons,  perhaps  from  the  sensory  nucleus  of  the 
trigeminal  nerve.  It  is  probably  "especially  concerned  with  the  feeding  reflexes 
of  the  snout  or  muzzle,  including  smell,  touch,  taste,  and  muscular  sensibility, 
a  physiologic  complex  which  Edinger  has  called  collectively  the  'oral  sense'  " 
(Herrick,  1918). 


CHAPTER  XVIII 


THE  CORTEX  AND  MEDULLARY  CENTER  OF  THE  CEREBRAL 

HEMISPHERE 

The  cerebral  cortex  forms  a  convoluted  gray  lamina,  covering  the  cerebral 
hemisphere,  and  varies  in  thickness  from  4  mm.  in  the  anterior  central  gyrus 
to  1.25  mm.  near  the  occipital  pole.  When  sections  through  a  fresh  brain  are 
examined  macroscopically,  the  cortex  is  seen  to  be  composed  of  alternating 
lighter  and  darker  bands,  the  light  stripes  being  produced  by  aggregations  of 
myelinated  nerve-fibers  (Fig.  212). 

Nerve-fibers.— In  addition  to  a  very  thin  superficial  white  layer  of  tangential 
fibers  there  are  in  most  parts  of  the  cerebral  cortex  two  well-defined  white  bands, 
the  inner  and  outer  lines  of  Baillarger 
(Figs.  212,  215).  These  two  bands  con- 
tain large  numbers  of  myelinated  nerve- 
fibers  running  in  planes  parallel  to  the 
surface  of  the  cortex.  In  the  region  of 
the  calcarine  fissure  only  the  outer  line 
is  visible;  but  this  is  very  conspicuous 
and  is  here  known  as  the  line  of  Gcnnari. 
Myelinated  fibers  enter  the  cortex  from 
the  white  center  in  bundles  that  in 
general  have  a  direction  perpendicular 
to  the  surface  of  the  cortex.  These 
bundles  radiate  into  each  convolution  from  its  central  white  core  and  separate 
the  nerve-cells  into  columnar  groups,  thus  giving  the  cortex  a  radial  striation 
(Fig.  215). 

Many  of  the  fibers  in  these  radial  bundles  are  eortieifugaL  representing  the 
axons  of  the  pyramidal  and  polymorphic  cells  of  the  cortex.  Within  the  medul- 
lar}- center  they  run  (1)  as  association  fibers  to  other  parts  of  the  cortex  of  the 
same  hemisphere,  (2)  as  commissural  fibers,  through  the  corpus  callosum  to  the 
opposite  hemisphere,  or  (3)  as  projection  fibers  to  the  thalamus  and  lower  .lying 
centers.  The  others  are  cortici petal  and  are  derived  in  part  from  the  thalamic 
radiation;  but  an  even  greater  number  of  them  are  the  terminal  portions  of  as- 

283 


Fig.  212. — Schematic  sections  of  cerebral 
gyri  showing  the  alternate  lighter  and  darker 
bands  which  compose  the  cerebral  cortex:  1 
shows  the  layers  as  seen  in  most  parts  of  the 
cerebral  cortex;  2,  the  layers  as  seen  in  the 
region  of  the  calcarine  fissure.  (Baillarger, 
Quain's  Anatomy.) 


284 


THK    NERVOUS    SYSTEM 


sociation  and  commissural  libers  from  other  parts  of  the  cortex.  Many  of  these 
fibers  end  in  the  most  superficial  stratum  of  the  cortex,  the  plexiform  layer,  where 
the  terminal  branches  of  the  apical  dendrites  of  the  pyramidal  cells  are  widely 
expanded  (Fig.  214).     Others  terminate  as  indicated  in  Fig.  213,  where  they 


Fig.  213. — From  the  anterior  central  gyrus  of 
the  human  cerebral  cortex,  showing  the  terminations 
of  corticipetal  fibers:  a,  b,  Afferent  fibers;  B,  dense 
network  produced  by  the  terminal  branches  of  such 
fibers.     Golgi  method.     (Cajal.) 


Fig.  214. — Nerve-cells  and  neuroglia 
from  the  cerebral  cortex:  A,  Neuroglia;  B, 
horizontal  cells  of  Cajal;  C,  pyramidal  cells; 
D,  cell  of  Martinotti;  E,  stellate  cell. 


are  seen  forming  a  close  network  of  unmyelinated  fibers.  Enmeshed  in  the 
dense  fiber  plexus  indicated  at  B,  Fig.  213,  are  the  pyramidal  cells  illustrated 
in  Layer  III  of  Fig.  215. 

The  nerve-cells  of  the  cortex  are  disposed  in  fairly  definite  layers  as  indicated 
in  Fig.  215.     We  may  enumerate  five  well-recognized  varieties:  (1)  the  pyra- 


[HE  CORTEX  AM)  MEDULLARY  CENTER  OF  mi  CEREBRAL  BEMISPHERE   285 

mi<lal,  (2)  the  stellate,  and  (3)  the  polymorphous  cells,  as  well  as  (4)  the  hori- 
zontal cells  of  Cajal,  and  (5)  the  cells  of  Martinotti. 

The  pyramidal  cells  are  the  most  numerous  and  are  classified  as  small, 
medium,  large,  and  giant  pyramidal  cells  (Fig.  215).  From  the  base  of  a  pyra- 
midal cell  body  an  axon  extends  toward  the  subjacent  white  matter,  giving 
off  collaterals  which  ramify  in  the  adjacent  cortex  (Figs.  23,  214,  C).  The  den- 
drites are  of  two  kinds:  a  large  apical  dendrite  and  numerous  smaller  ones  at- 
tached to  the  base  and  sides  of  the  pyramid.  The  apical  dendrite  appears  as  an 
extension  of  the  cell  bod}'  and  is  directed  toward  the  surface  of  the  cortex,  near 
which  it  ends  in  spreading  branches.  Its  length  varies  with  the  depth  of  the 
cell  body  from  the  surface.  To  an  even  greater  extent  than  other  dendrites  it  is 
provided  with  short  thorny  processes  called  "spines"  or  "gemmules."  These 
are  supposed  by  some  to  effect  contact  with  neighboring  axonic  ramifications 
and  to  be  retractile.  Upon  retraction  of  these  gemmules,  conduction  across 
the  synapse  would  be  interrupted  for  the  time  being;  and  one  might  explain 
the  varying  sensory  thresholds  of  an  individual  in  sleep  or  during  attention  by 
the  varying  degree  of  expansion  of  the  gemmules.  But  as  yet  no  satisfactory 
evidence  in  support  of  the  theory  has  been  presented. 

The  stellate  cells  are  also  known  as  granules.  They  are,  for  the  most  part, 
of  small  size,  and  their  short  axons  branch  repeatedly  and  terminate  in  the 
neighborhood  of  the  cell  of  origin.  That  is  to  say,  they  are  cells  of  Golgi's 
Type  II.  Although  they  occur  in  most  layers  of  the  cortex,  they  are  especially 
numerous  in  the  fourth  stratum,  which  is  accordingly  designated  as  the  layer 
of  small  stellate  cells  (Figs.  214,  E;  215). 

The  cells  of  Martinotti,  which  are  also  found  in  most  of  the  cortical  strata, 
have  this  as  their  distinguishing  characteristic,  that  their  axons  are  directed 
toward  the  surface  of  the  cortex  and  ramify  in  the  superficial  layer  (Fig.  214,  D). 

The  horizontal  cells  of  Cajal,  which  are  present  only  in  the  superficial  layer, 
are  fusiform,  with  long  branching  dendrites  directed  horizontally.  Their  axons 
are  long  and  form  tangential  myelinated  fibers  in  the  superficial  layer  (Fig.  214,5). 

Polymorphous  cells,  fusiform  or  angular  in  shape,  are  found  in  the  deepest 

stratum  of  the  cortex  (Figs.  214,  215).     Their  axons  enter  the  subjacent  white 

matter. 

CELL  AND  FIBER  LAMINATION 

The  size  and  type  of  cells  found  in  the  cortex  vary  at  different  depths  from 
the  surface,  that  is  to  say,  the  cells  are  disposed  in  fairly  definite  layers.  As 
already  indicated,  many  of  the  myelinated  fibers  are  arranged  in  bands  parallel 


286 


THE    NERVOUS 


to  the  -urface.     By  means  of  this  cell  and  fiber  lamination  Brodmann  (1909) 

gnizes  six  layers  in  the  cerebral  cortex  fTig.  215).     Other  authors,  notably 

Campbell  (1905)  and  Cajal  (1906),  number  these  layers  somewhat  differently. 

Moreover,  the  arrangement  varies  in  different  parts  of  the  cortex.     In  certain 


.',•  • 


■  mmkmm 


Fig.  215. — Diagram  of  the  structure  of  the  cerebral  cortex:  7,  Molecular  layer;  II,  layer  of 
small  pyramidal  cells;  III,  layer  of  medium-sized  and  large  pyramidal  cells;  IV,  layer  of  small 
stellate  cells;  V,  deep  layer  of  large  pyramidal  cells;  VI,  layer  of  polymorphic  cells;  ja\  band  of 
Bechterew;  _/,  outer  band  of  Baillarger;  56,  inner  band  of  Baillarger.     (Brodmann.) 

regions  one  or  more  of  the  strata  may  be  reduced,  enlarged  or  subdivided,  but 
the  arrangement  in  most  parts  is  substantially  like  that  illustrated.  The  six 
layers  are  as  follows: 

1.  The  molecular  layer  (plexiform  layer,  lamina  zonalis)  is  the  most  super- 
ficial.    It  contains  the  superficial  band  of  tangential  myelinated  fibers  and  many 


Mil:    (  okll  \     WD    Ml  IHI.I.AKV    (INTER   OF     Nil     CEREBRAL  HEMISPHERE       287 

neuroglia  cells.  The  nerve-cells  arc  of  two  kinds:  (1)  horizontal  cells  of  Caial, 
and  (2)  cells  of  Golgi's  Type  II.  Within  this  layer  ramify  the  terminal  branches 
of  the  apical  dendrites  from  tin-  pyramidal  cells  of  the  deeper  layers. 

2.  The  layer  of  small  pyramidal  cells  (lamina  granulans  externa)  contains  a 
Urge  number  of  small  nerve  cells.  Most  of  these  are  .mall  pyramids  with  axons 
running  to  the  white  center  of  the  hemisphere.  Other.-  belong  to  the  diort- 
axoned  group  (Golgi's  Type  II). 

3.  The  layer  of  medium-sized  and  large  pyramidal  cells  (lamina  pyramidalis  I 
may  be  subdivided  into  two  substrata,  the  more  superficial  stratum  containing 
chiefly  medium-sized  pyramids  and  the  deeper  one  chiefly  large  pyramids.  There 
are  also  present  cells  of  Golgi's  Type  II  and  cells  of  Martinotti.  According  to 
Cajal  (1900-1906)  and  Campbell  (1905),  it  is  within  this  layer  that  the  outer 
stripe  of  Baillarger  is  located,  but  Brodmann  places  this  line  in  the  next  layer. 

4.  The  layer  of  small  stellate  cells  (lamina  granularis  interna)  is  characterized 
by  the  presence  of  a  large  number  of  small  multipolar  cells  with  short  axons 
(Golgi's  Type  II).  Scattered  among  these  are  small  pyramids.  Brodmann 
places  the  outer  line  of  Baillarger  in  this  stratum. 

5.  The  deep  layer  of  large  pyramidal  cells  (lamina  ganglionaris)  contains  the 
largest  cells  of  the  cortex.  In  the  motor  region  these  are  known  as  the  giant 
pyramidal  cells  of  Betz  and  give  origin  to  the  fibers  of  the  corticospinal  tract. 
The  apical  dendrites  of  these  cells  are  very  long  and,  like  those  of  the  more  super- 
ficial pyramidal  cells,  reach  and  ramify  within  the  molecular  layer.  Smaller 
cells,  both  of  the  pyramidal  and  short-axoned  type,  are  also  present.  The 
horizontal  fibers  of  Baillarger's  internal  line  are  found  in  this  layer  in  most  of 
the  cortical  areas. 

6.  The  layer  of  polymorphic  cells  (lamina  multiformis)  contains  irregular 
fusiform  and  angular  cells,  the  axons  of  which  enter  the  subjacent  white  matter. 

Cortical  Areas. — The  six  layers  of  the  cortex  are  arranged  in  most  regions 
essentially  as  shown  in  Fig.  215.  But  each  of  more  than  forty  areas  presents  its 
own  characteristic  variation  in  the  structure,  thickness,  and  arrangement  of 
the  cellular  layers,  in  the  thickness  of  the  cortex  as  a  whole,  in  the  number  of 
afferent  and  efferent  myelinated  fibers,  and  in  the  number,  distinctness,  and  posi- 
tion of  the  white  striae.  On  the  basis  of  such  differences  the  entire  cortex  has 
been  subdivided  into  structurally  distinct  areas.  Maps  of  such  cortical  areas 
have  been  furnished  by  Brodmann  (1909),  Campbell  (1905),  and  Elliot  Smith 
(1907);  and  while  these  vary  in  detail,  they  agree  in  their  larger  outlines.  The 
existence  and  general  boundaries  of  these  regions  are  now  well  established;  and 


288 


THE    NERVOUS    SYSTEM 


as  a  result  of  experimental  and  pathologic  research  it  is  known  that  specific 
differences  b  function  are  correlated  with  these  differences  in  structure. 

The  maps  of  the  cortical  areas  furnished  by  Brodmann  are  reproduced  in 
Figs.  216  and  217.    He  recognizes  eleven  general  regions,  and  each  of  these  may 


Fig.  216. 


20 
Fig.  217. 

Figs.  216  and  217.-Areas  of  the  human  cerebral  cortex  each  of  which  possesses  a  distinctive 
structure:  Fig.  216,  lateral  view;  Fig.  217,  medial  view.     (Brodmann.) 

be  subdivided  into  smaller  areas  on  the  basis  of  characteristic  differences  in 
structure.  Some  of  these  differences  are  visible  to  the  naked  eye  and  have 
been  represented  in  Fig.  218. 


nil     CORTEX     \\l>    MEDULLARY.    CENTEB    "I      un     CEREBRAL    EEMISPHER] 

Myelination.  The  fibers  in  the  various  parts  of  the  cortex  acquire  their 
myelin  sheaths  at  different  times.  On  this  ba>i>  Flech  [g  (1896  Identified 
thirty-six  areas,  which  arc  numbered  in  Fig.  21()  in  the  order  of  myelination.  I [e 
recognizes  three  main  groups:  primary  (Nos.  1  to  I2)s  intermediate  (Nos.  13 


I  ig  218. — Diagram  showing  the  differences  in  thickness  and  in  the  arrangemenl  (if  t  In-  lighter 
ami  darker  bands  in  the  human  cerebral  cortex  in  different  regions  as  seen  with  the  naked  eye: 
.1,  Motor  cortex  from  anterior  central  gyrus;  B,  sensory  cortex  from  the  posterior  central  gyrus; 
(',  \isiial  cortex  from  the  region  of  the  calcarine  fissure;  D,  auditory  cortex  from  the  anterior 
transverse  temporal  gyrus.      (Redrawn  after  Elliot  Smith.) 

to  28),  and  late  (Nos.  28  to  36).  According  to  Flechsig,  the  primary  areas, 
which  are  myelinated  at  birth,  are  projection  centers  and  receive  the  sensory 
radiation  from  the  thalamus;  while  the  other  parts  of  the  cortex,  not  being  pro- 
vided with  projection  fibers,  serve  only  as  association  centers.     He  believed  that 


Fig.  219. — Lateral  view  of  the  human  cerebral  hemisphere,  showing  the  cortical  areas  as 
outlined  by  Flechsig  on  the  basis  of  differences  in  the  time  of  myelination  of  their  nerve-fibers. 
The  primary  areas  (first  to  become  well  myelinated)  are  cross-hatched;  the  intermediate  are 
indicated  by  vertical  lines;  the  late  areas  are  unshaded.     (Lewandowsky.) 

myelination  of  nerve-fibers  takes  place  in  the  order  of  conduction,  that  is,  the 
sheaths  are  developed  first  on  the  afferent  fibers,  reaching  the  cortex  from  the 
thalamus,  and  later  on  the  association  fibers,  linking  the  various  areas  together. 
According  to  this  conception  fibers  of  like  function  tend  to  become  myelinated 
19 


290 


THE    NERVOUS    SYSTEM 


at  the  same  time.  Much  of  Flechsig's  work  has  failed  to  stand  the  test  of  rigid 
examination.  It  is  now  known  that  practically  all  regions  of  the  cortex,  in- 
cluding those  designated  by  him  as  association  centers,  are  connected  with  the 
thalamus  or  lower  lying  centers  by  afferent  or  efferent  projection  fibers.  It 
has  been  shown  that  the  more  mature  areas  fade  off  gradually  into  those  whose 
differentiation  is  less  advanced,  and  that  sharply  outlined  zones  such  as  are 
indicated  in  his  figures  do  not  exist.  Nevertheless,  it  is  true  that  the  regions 
designated  by  him  as  primary  areas,  though  not  sharply  outlined  by  this  method 
from  the  surrounding  cortex,  do  mature  first,  and  the  myelination  spreading 
from  these  areas  reaches  its  completion  last  in  those  areas  included  in  his  late 
group  (Brodmann,  1910).  The  primary  areas  include  the  region  surrounding 
the  central  fissure,  the  region  around  the  calcarine  fissure,  a  portion  of  the 
superior  temporal  gyrus,  and  a  part  of  the  hippocampal  gyrus.  These  areas 
are  associated  with  especially  important  projection  tracts  and  may  properly 
be  spoken  of  as  projection  centers. 

CORTICAL  OR  CEREBRAL  LOCALIZATION 

In  opposition  to  the  crude  conceptions  of  the  localization  of  cerebral  functions 
introduced  by  Gall  (1825),  which  formed  the  basis  for  phrenology,  the  French 
physiologist  Florens  maintained  the  doctrine  that  all  parts  of  the  cerebrum  are 
functionally  equivalent.  In  1861  Broca  demonstrated  that  destruction  of  the 
left  third  frontal  convolution  may  result  in  a  loss  of  ability  to  speak;  and  nine 
years  later  Fritsch  and  Hitzig  (1870)  discovered  that  electric  excitation  of  the 
cortex  in  the  region  of  the  central  sulcus  will  elicit  movements  from  muscles  of 
the  opposite  side  of  the  body.  These  observations,  confirmed  and  extended 
by  many  observers,  definitely  proved  that  certain  cortical  areas  possess  spe- 
cialized functions.  Physiologic  and  pathologic  researches  have  served  to  out- 
line a  number  of  these  with  considerable  precision,  and  it  is  possible  to  identify 
them  with  regions  of  characteristic  cell  and  fiber  lamination.  In  this  way  evi- 
dence derived  from  histologic  studies  reinforces  that  drawn  from  physiology  and 
pathology. 

The  motor  projection  center  is  located  in  the  anterior  wall  of  the  central  sulcus, 
in  the  adjacent  part  of  the  anterior  central  gyrus,  and  in  that  part  of  the  para- 
central lobule  which  lies  rostral  to  the  continuation  of  the  central  sulcus  on  the 
medial  surface  of  the  hemisphere  (Figs.  220,  221).  It  coincides  fairly  closely 
with  Area  4  of  Brodmann's  charts  (Figs.  216,  217).  This  is  the  center  from  which 
the  impulses  initiating  voluntary  movements  on  the  opposite  side  of  the  body 


THE  CORTEX  AND  MEDULLARY  CENTER  OF  Mil.  CEREBRAL  EEMISPHERE  2gi 

descend  to  the  motor  nuclei  of  the  cerebrospinal  nerves,  it  is  subdivided  into 
areas,  eaeli  of  which  controls  the  muscles  moving  a  given  part  of  the  opposite 
hall"  of  the  body;  an<l  tiiese  art'  arranged  in  inverted  order,  beginning  with  the 
center  for  movement  of  the  toes  near  the  dorsal  border  of  the  hemisphere,  and 
ending  with  that  for  the  face  at  the  lower  end  of  the  anterior  central  gyrus  (Fig. 
236). 

The  structure  of  the  motor  cortex  is  characteristic.  Here  the  gray  matter 
reaches  the  maximum  thickness,  the  lines  of  Uaillarger  are  broad  and  diffused 
(Fig.  218).  The  fifth  layer  contains  the  giant  pyramidal  cells  of  Betz,  from 
which  arise  the  libers  of  the  corticospinal  and  corticobulbar  tracts.  These 
cells  undergo  chromatolysis  when  these  motor  tracts  are  cut;  and  when  the  motor 
cortex  is  destroyed  the  tracts  degenerate  (Holmes  and  May,  1909). 


Motor  projection  center 


Somesthetic  area 

.  1  ud  i  lory    re- 
ceptive center 


Motor  projection  center 


Visual  receptive  center 

Fig.  220. — Diagram  of  the  cortical  pro- 
jection centers  on  the  lateral  aspect  of  the 
cerebral  hemisphere. 


Somesthetic  area 


Visual  re- 
Olfactory  center  cePtive  center 

Fig.  221. — Diagram  of  the  cortical  pro- 
jection centers  on  the  medial  aspect  of  the 
cerebral  hemisphere. 


The  motor  cortex  of  the  chimpanzee  corresponds  in  its  arrangement  with 
that  of  man;  and  by  the  electric  excitation  of  its  different  portions  muscular 
contractions  can  be  excited  in  the  corresponding  parts  of  the  opposite  side  of 
the  body  (Griinbaum  and  Sherrington,  1903).  In  addition,  there  is  an  area 
farther  forward  in  the  frontal  lobe  the  stimulation  of  which  produces  conjugate 
movements  of  the  eyes.  A  similar  center  for  the  conjugate  deviation  of  the 
head  and  eyes  is  situated  in  the  posterior  part  of  the  middle  frontal  gyrus  in 
man.  It  is  probable,  however,  that  this  motor  center  is  of  a  different  kind 
from  those  found  in  the  anterior  central  gyrus,  from  which  all  of  the  fibers  of 
the  pyramidal  system  are  believed  to  take  their  origin  (Fig.  236). 

The  sensory  projection  centers  are  the  areas  within  which  terminate  the 
sensory  projection  fibers.     We  have  learned  to  locate  such  centers  for  vision. 


292  THE    NERVOUS    SYSTEM 

bearing,  smell,  and  the  general  sensations  from  the  surface  of  the  body  and  the 
deeper  tissues.  The  latter  region,  known  as  the  common  sensory  or  somesthetic 
area,  is  located  in  the  posterior  central  gyrus  (Areas  1,  2,  and  3  of  Brodmann). 
It  receives  fibers  belonging  to  the  thalamic  radiation  from  the  lateral  nucleus  of 
the  thalamus  and  representing  neurons  of  the  third  order  in  the  afferent  paths 
from  the  skin,  muscles,  joints,  and  tendons. 

The  most  conclusive  evidence  of  the  sensory  function  of  the  posterior  central 
gyrus  is  furnished  by  Cushing's  (1909)  observations  on  the  electric  excitability 
of  the  human  cerebral  cortex.  These  tests  were  made  on  unanesthetized  patients 
in  the  course  of  operations  for  brain  tumors.  Stimulation  of  the  cortex  within 
the  posterior  central  gyrus  caused  the  patients  to  experience  cutaneous  sensa- 
tions, which  seemed  to  come  from  the  skin  of  the  hand,  but  did  not  elicit  any 
motor  responses;  while  in  these  same  cases  stimulation  of  the  anterior  central 

Calcarine  fissun  — ~       ~~i*i«»^ 

i^v^**%o  **** Tangential  fibers 

Stria  of  Gennari 

Transition  between  striate --^Ijj^S  fteft,  u^^M^-    White  center 

and  peri  striate  areas 

Cuneus  


Fig.  222. — Section  through  the  most  rostral  part  of  the  cuneus.     Pal-Weigert  method. 

gyrus  gave  rise  to  no  sensations,  but  did  call  forth  muscular  contractions.  On 
the  other  hand,  Head  (1918),  in  a  recent  study  of  "Sensation  and  the  Cerebral 
Cortex,"  would  include  in  the  somesthetic  area  the  anterior  as  well  as  the  posterior 
central  convolution,  and  also  the  anterior  part  of  the  superior  parietal  lobule 
and  the  angular  gyrus.  This  study  shows,  perhaps  better  than  any  other  work, 
how  intricate  and  difficult  the  problem  of  cortical  localization  really  is  and  how 
far  we  are  from  an  ultimate  solution. 

The  visual  receptive  center  is  located  in  the  cortex  forming  the  walls  of  the 
calcarine  fissure  and  in  the  adjacent  portions  of  the  cuneus  and  the  lingual 
gyrus  (Figs.  217,  221).  Rostral  to  the  point  where  the  calcarine  is  joined  by  the 
parieto-occipital  fissure  the  visual  cortex  is  located  only  along  the  ventral  side 
of  the  former.  Sometimes  the  center  may  extend  around  the  occipital  pole  on 
to  the  lateral  surface  of  the  brain  (Fig.  216,  Area  17).  The  structural  peculiar- 
ities of  the  visual  cortex  are  very  evident.     It  is  not  more  than  one-half  as  thick 


Till:    CORTEX     \\1>    MEDULLAR?    CENTEB    01      Mil     CEREBRAL   HEMISPHERE       293 

as  the  motor  cortex,  and  the  outer  line  of  Baillarger  is  greatly  in<  reased  in  thn  k 
ness  and  known  as  the  line  of  German  (Fig.  218,  C).    Because  of  the  prominence 
of  this  line  the  region  i>  known  as  the  area  striata.     It  is  surrounded  \>\  cortex 
of  quite  different  structure;  and  uowhere  « an  the  diflerem  es  in  adja<  <  al  <  orti<  al 

areas  he  better  illustrated  than  at  its  border,  where  the  prominent  line  of  ( lennari 
i-  -een  to  terminate  abruptly  (Fig.  222).  The  fibers  of  the  opti<  radiation  from 
tin-  pulvinar  and  lateral  geniculate  body  terminate  in  the  visual  projection  center. 
These  fibers  carry  impulses  from  the  temporal  side  of  the  corresponding  retina 
and  the  nasal  side  of  the  opposite  one.  The  visual  cortex  of  one  hemisphere, 
therefore,  receives  impressions  from  the  objects  on  the  opposite  side  of  the  line 
of  vision  (Figs.  162,  163). 

The  auditory  receptive  center  is  located  in  the  anterior  transverse  temporal 
gyrus,  which  lies  buried  in  the  floor  of  the  lateral  sulcus.  The  area  comes  to 
the  surface  near  the  middle  of  the  dorsal  border  of  the  superior  temporal  gyrus 
(Fig.  220).     It  receives  the  auditory  radiation  from  the  medial  geniculate  body. 

The  olfactory  receptive  center  is  located  in  the  uncus  and  adjacent  portions 
of  the  hippocampal  gyrus  (principal  olfactory  area  of  Cajal).  Within  it  ter- 
minate the  fibers  of  the  lateral  olfactory  stria.  They  form  a  rather  thick  layer 
of  tangential  fibers  on  its  surface,  which  increases  the  thickness  of  the  plexiform 
layer. 

Association  Centers. — It  will  be  seen  that  the  sensory  and  motor  projection 
centers  occupy  only  a  small  part  of  the  entire  area  of  the  cortex.  The  remaining 
parts  are  connected  with  these  centers  by  association  fibers  and  are  known  as 
association  centers.  Each  area  of  sensory  projection  is  surrounded  by  a  zone 
closely  linked  up  with  it  by  such  fibers,  and  therefore  probably  under  the  dom- 
inating influence  of  the  particular  sensory  impulses  reaching  that  projection 
center.  Their  positions  are  indicated  by  lighter  shading  in  Figs.  220  and  221. 
Campbell  (1905)  has  applied  to  them  the  designations  "audito-psychic"  and 
"visuo-psychic  fields"  (Figs.  223,  224).  The  same  author  has  designated 
the  portion  of  the  frontal  cortex  immediately  rostral  to  the  motor  projection 
center  the  "intermediate  precentral  area,"  and  is  of  the  opinion  it  is  especially 
concerned  with  the  "execution  of  complex  movements  of  an  associated  kind, 
of  skilled  movements,  and  of  movements  in  which  consciousness  or  volition  takes 
an  active  part."  There  still  remains  more  than  half  of  the  cortical  area,  in- 
dicated in  white  in  Figs.  220  and  221,  which  is  probably  less  intimately  related 
to  any  particular  projection  center.  The  fact  that  the  increased  size  of  the 
human  cerebral  hemisphere  over  that  of  the  higher  apes  is  due  to  the  much 


294 


THE    NERVOUS    SYSTEM 


greater  development  of  the  association  centers  in  man,  suggests  that  these  are  of 
especial  significance  for  the  higher  intellectual  functions. 


Vi.iuo-seiisojy 


J- 


Fig.  224. 
Figs.  223  and  224.— Areas  of  the  human  cerebral  cortex  each  of  which 

structure.     (Campbell.) 


possesses  a  distinctive 


In  the  present  state  of  our  knowledge  of  cortical  activity  and  its  relation  to 
consciousness  it  is  the  part  of  wisdom  to  be  very  conservative  in  locating  any 
mental  faculty  or  fraction  of  our  conscious  experience  in  any  particular  part  of 


Nil:  CORTEX  AM)  MEDULLARY  CENTEB  OF  Mil.  CEREBRAL  BEMISPHERE   295 

the  cerebral  cortex.     We  know  upon  which  areas  the  auditory,  visual,  and  olfa< 

tory  impulses  impinge,  and  less  accurately  that  in  which  the  thalamic  radiation. 
mediating  general  bodily  sensibility,  terminates.  Destruction  of  these  areas 
causes  impairment  or  loss  of  the  corresponding  sensations  with  reference  to  the 
opposite  side  of  the  body  or  the  opposite  half  of  the  field  of  vision.  Total  loss 
of  cutaneous  sensibility  even  within  circumscribed  areas  never  results  from  cor- 
tical lesions;  and  it  seems  probable  that  the  thalamic  centers  are  in  themselves 
sufficient  for  a  certain  low  grade,  non-discriminative  consciousness  or  awareness 
of  cutaneous  stimulation.  This  is  particularly  true  of  painful  sensations,  which 
seem  to  be  for  the  most  part  of  thalamic  origin  (Head,  1918).  Furthermore, 
the  various  parts  of  the  cerebral  cortex  are  so  intimately  linked  together  by  as- 
sociation fibers  that  when  afferent  impulses  reach  a  given  projection  center  they 
must  not  only  activate  this  center,  but  be  propagated  to  other  parts  of  the  cortex 


Motor  speech  ccnttr 


Auditory  speech  center   Visual  sPeech  ccnlcr 
Fig.  225. — The  cortical  areas  especially  concerned  with  language. 


as  well.  In  view  of  these  facts  it  is  best  to  express  the  known  facts  of  cortical 
localization  in  terms  of  the  relation  of  particular  areas  to  the  known  projection 
fiber  systems. 

Aphasia. — Some  idea  of  the  significance  of  the  so-called  association  centers 
may  be  obtained  from  a  study  of  the  group  of  speech  defects  included  under  the 
term  "aphasia."  In  right-handed  individuals  these  result  from  lesions  in  the 
left  hemisphere.  Destruction  of  the  triangular  and  opercular  portions  of  the 
inferior  frontal  gyrus  usually  causes  loss  of  ability  to  carry  out  the  coordinated 
movements  required  in  speaking,  but  does  not  impair  the  ability  to  move  the 
tongue  or  lips  (Fig.  225).  This  defect  is  known  as  motor  aphasia.  Broca's 
center,  as  this  particular  part  of  the  cortex  is  sometimes  called,  is  located  in 
Campbell's  intermediate  precentral  area;  and  motor  aphasia  serves  as  a  good 
illustration  of  the  importance  of  the  entire  intermediate  precentral  area  for  the 


THE    NERV01  S    -',-11  M 

ution  of  -killed  volitional  movements  of  an  associated  kind.  In  the  same 
way.  after  a  lesion  in  the  posterior  part  of  the  left  superior  temporal  gyrus, 
the  patient  may  hear  the  spoken  word,  but  no  longer  comprehend  its  meaning. 
This  is  sensory  aphasia  or  word  deafness.  Word  blindness,  the  inability  to  under- 
stand the  printed  or  written  language,  although  there  is  no  impairment  of  vision, 
may  result  from  lesions  in  the  angular  gyrus.  These  three  areas  are  often  spoken 
<>!'  ;i-  speech  centers  and  are  closely  united  together  by  association  fibers.  In 
fact,  it  is  not  altogether  clear  to  what  extent  such  defects  as  those  mentioned 
above  arc  dependent  upon  the  destruction  of  these  association  tracts  which  lie 
subjacent  to  the  speech  centers. 

THE  MEDULLARY  CENTER  OF  THE  CEREBRAL  HEMISPHERE 

The  medullary  center  of  the  cerebral  hemi>phere  underlies  the  cortex  and 
-epa rates  it  from  the  lateral  ventricle  and  corpus  striatum.  It  varies  greatly 
in  thickness,  from  that  of  the  thin  lamina  separating  the  insula  and  the  claus- 
trum  (Fig.  191 )  to  that  of  the  massive  centrum  semiovale  (Fig.  174).  The 
myelinated  nerve-fibers  of  which  it  is  composed  are  of  three  kinds,  namely,  as- 
sociation libers,  projection  fibers,  and  commissural  fibers. 

Commissural  Fibers. — As  was  stated  in  Chapter  XV,  there  are  three  com- 
missures joining  together  the  cerebral  hemispheres.  Of  these,  the  corpus  callo- 
sum  is  by  far  the  largest  and  its  radiation  contributes  largely  to  the  bulk  of  the 
centrum  semiovale  (Fig.  174).  The  fibers  which  compose  it  arise  in  the  various 
parts  of  the  neopallium  of  each  hemisphere;  they  are  assembled  into  a  broad 
compact  plate  as  they  cross  the  median  plane,  and  then  spread  out  again  to 
terminate  in  the  neopallium  of  the  opposite  side.  As  they  spread  through  the 
centrum  semiovale  they  form  the  radiation  of  the  corpus  callosum.  Some  cor- 
tical areas  are  better  supplied  with  these  fibers  than  others,  few.  if  any.  being 
associated  with  the  visual  cortex  about  the  calcarine  fissure  (Van  Valkenburg, 
1913).  The  majority  of  the  callosal  fibers  do  not  connect  together  symmetric 
portion-  of  the  cortex;  but.  after  crossing  the  median  plane,  the  fibers  from  a 
given  point  in  one  hemisphere  spread  out  to  many  parts  of  the  opposite  side. 
The  anterior  and  hippocampal  commissures  connect  portions  of  the  rhinencephalon 
in  one  hemisphere,  with  similar  parts  on  the  opposite  side.  The  anterior  com- 
missure  connects  together  by  its  rostral  part  the  two  olfactory  bulbs  and  by  its 
caudal  part  the  two  pyriform  areas  (Figs.  187,  194.  195).  The  hippocampal 
commissure  is  composed  of  fibers  which  join  together  the  two  hippocampi  by 
way  of  the  fimbria.*  and  the  psalterium. 


Mil     CORTEX     Wl>    MEDULLAR?    CENTER    OF     llll     CEREBRAL    HEMISPHER] 


'97 


Projection  Fibers.  Many  of  the  fibers  of  the  medullary  white  <  enter  <  oiu 
the  cerebral  cortex  with  the  thalamus  and  lower  lying  portions  of  the  nervous 
system.  These  are  known  as  projection  fibers,  and  may  be  divided  into  two 
groups  according  as  they  convey  impulses  to  or  from  the  cerebral  cortex.  The 
corticipetal  or  afferent  projection  fibers  include  the  following:  (1)  the  optu  radio 
Hon,  which  arises  in  the  pulvinar  of  the  thalamus  and  the  lateral  geniculate 
\nn\y  and  ends  in  the  visual  cortex  about  the  calcarine  fissure  (Fig.  221);  (2)  the 
auditory  radiation,  which  arises  in  the  medial  geniculate  body  and  terminates  in 
the  auditory  cortex  of  tin-  anterior  transverse  temporal  gyrus;  (3)  the  thalamic 
radiation  which  unites  the  lateral  nucleus  of  the  thalamus  with  various  parts  of 
the  cerebral  cortex,  and  which  forms  the  ventral,  frontal,  and  parietal  -talks  of 
the  thalamus  (Fig.  105V  The  fibers  of  the  parietal  stalk  include  the  sensory 
fibers  to  the  somesthetic  cortex  in  the  posterior  central  gyrus.  The  lateral  ol- 
factory stria,  which  conveys  impulses  from  the  olfactory  bulb  to  the  pyriform 
area,  is  not  a  projection  system  in  the  strict  sense  of  the  word,  since  it  begins 
and  ends  within  the  telencephalon. 

Efferent  projection  fibers  convey  impulses  from  the  cerebral  cortex  to  the 
thalamus,  brain  stem,  and  spinal  cord.  They  represent  the  axons  of  pyramidal 
cells.  The  most  important  groups  are  those  of  the  corticospinal  and  corticobulbar 
tracts,  which  together  form  the  great  motor  or  pyramidal  system.  These  fibers 
begin  in  the  motor  cortex  of  the  anterior  central  gyrus  as  axons  of  the  giant  cells 
of  Betz.  Entering  the  white  medullary  center  of  the  hemisphere,  they  are  as- 
sembled in  the  corona  radiata  (Fig.  194)  and  enter  the  internal  capsule  (Fig. 
88).  Their  course  beyond  this  point  has  been  traced  in  the  preceding  chapters. 
They  convey  impulses  to  the  primary  motor  neurons  of  the  opposite  side  of  the 
brain  stem  and  spinal  cord.  Another  important  group  of  corticifugal  fibers  is 
contained  in  the  corticopontine  tracts.  Of  these  there  are  two  main  strands. 
The  frontopontine  tract  consists  of  fibers  which  begin  as  axons  of  cells  in  the  cortex 
of  the  frontal  lobe,  traverse  the  centrum  semiovale.  corona  radiata,  frontal  part 
of  the  internal  capsule  and  medial  one-fifth  of  the  basis  pedunculi.  and  finally 
terminate  in  the  nuclei  pontis.  The  temporopontine  tract  has  a  similar  origin 
from  the  cortical  cells  of  the  temporal  lobe  and  possibly  of  the  occipital  lobe  also, 
passes  through  the  sublenticular  part  of  the  internal  capsule  and  lateral  one- 
fifth  of  the  basis  pedunculi,  and  finally  terminates  in  the  nuclei  pontis  (Figs. 
88,  106).  The  ascending  thalamic  radiation  is  paralleled  by  descending 
corticothalamic  fibers,  which  should  be  included  among  the  efferent  projection 
systems,  although  their  physiologic  significance  is  not  fully  understood.     Similar 


298 


THE    NERVOUS    SYSTEM 


efferent  fibers  are  contained  in  the  optic  radiation.  They  arise  in  the  cortex 
about  the  calcarine  fissure  and  terminate  in  the  pulvinar.  lateral  geniculate 
body,  and  superior  colliculus  of  the  corpora  quadrigemina  (Fig.  162).  A  corti- 
coruhral  tract  descend?-  from  the  frontal  lobe  through  the  posterior  limb  of  the 
internal  capsule  to  end  in  the  red  nucleus  of  the  mesencephalon.  There  do  not 
appear  to  be  any  strictly  corticostriate  fibers,  but.  according  to  Cajal  (1911), 
collateral-  from  the  corticospinal  fibers  are  given  off  to  the  corpus  striatum. 
The  efferent  projection  tracts  which  we  have  considered  all  have  their  origin  in 
the  neopallium. 

There  are  several  projection  tracts  from  the  rhincncephaJon,  and  of  these  the 
most  important  is  the  fornix.     The  fibers  of  this  fascicle  take  origin  in  the  hip- 


Fig.  226. — Some  of  the  important  association  bundles  projected  upon  the  medial  aspect  of  the 
cerebral  hemisphere.     (Sobotta-McMurrich.) 


pocampus,  follow  an  arched  course  already  described,  and.  entering  the  dien- 
cephalon.  terminate  in  part  in  the  mammillary  body  and  in  part  in  the  teg- 
mentum of  the  brain  stem  (Fig.  205). 

The  frontal  olfactory  projection  tract  arises  from  the  gray  matter  of  the  ol- 
factory peduncle  and  the  lateral  olfactory  gyrus.  It  enters  the  brain  stem  and 
terminates  in  the  pons  and  the  medulla  oblongata  (Fig.  211). 

Association  Fibers. — The  various  parts  of  the  cortex  within  each  hemisphere 
are  bound  together  by  association  fibers  of  varying  length.  The  short  associa- 
tion fibers  are  of  two  kinds:  (1)  those  which  run  in  the  deeper  part  of  the  cortex 
and  are  designated  as  intracortical,  and  (2)  those  just  beneath  the  cortex,  which 
are  known  as  the  subcortical  fibers.  The  greater  number  of  these  subcortical 
association  fibers  unite  adjacent  gyri.  curving  in  U-shaped  loops  beneath  the 


THE    CORTEX    AND    MEDULLAR?    CENTER    OF    Mil     CEREBRAL    BEMISPHER] 

intervening  sulci,  and  are  accordingly  often  designated  as  annate  fibei      i 
226).    Others  unite  somewhat  more  widely  separated  gyri.    The  Un  lotion 

fibers  form  bundles  of  considerable  size,  deeply  situated  in  the  medullary  center 
of  the  hemisphere,  and  unite  widely  separated  cortical  area-.  'I  lure  are  five 
of  these  which  may  be  readily  displayed  by  dissection  of  the  human  cerebral 
hemisphere,  namely,  the  uncinate,  inferior  occipitofrontal,  inferior  longitudinal, 
and  superior  longitudinal  fasciculi,  and  the  cingulum.  Another,  known  as  the 
fasciculus  occipitofrontal^  superior,  is  less  easily  displayed. 

The  cingulum  is  an  arched  bundle  which  partly  encircles  the  corpus  callosum 
not  far  from  the  median  plane  (Figs.  174,  226).  It  begins  ventral  to  the  n»trum 
of  the  corpus  callosum,  curves  around  the  genu  and  over  the  dorsal  surface  of 


Optic  radiation 


External  capsule  ami  leniiform  nucleus 
Corona  radiata         ;  Frontal  lobe 


--Fas.  occipitofrontalis 
inferior 
'Fas.  uncinatus 

-Temporal  lobe 


Fig.  227. — Lateral  view  of  a  dissection  of  a  human  cerebral  hemisphere.  The  dorsal  part 
of  the  hemisphere  has  been  cut  away.  On  the  lateral  side  the  insula,  opercula,  and  adjacent  parts 
have  been  removed. 


that  commissure  to  the  splenium,  and  then  bends  ventrally  to  terminate  near  the 
temporal  pole.  It  is  closely  related  to  the  gyrus  cinguli  and  the  hippocampal 
gyrus  and  is  composed  for  the  most  part  of  short  fibers,  which  connect  the  various 
parts  of  these  convolutions. 

The  uncinate  fasciculus  connects  the  orbital  gyri  of  the  frontal  lobe  with  the 
rostral  part  of  the  temporal  lobe.  It  is  sharply  bent  on  itself  as  it  passes  over 
the  stem  of  the  lateral  fissure  of  the  cerebrum  (Figs.  227,  228).  The  inferior 
longitudinal  fasciculus  is  a  large  bundle  which  runs  through  the  entire  length  of 
the  temporal  and  occipital  lobes  (Fig.  226).  It  connects  the  occipital  pole, 
the  cuneus,  and  other  parts  of  the  occipital  lobe  with  the  temporal  cortex,  ex- 
tending as  far  forward  as  the  temporal  pole.     According  to  Curran  (1909)  the 


3°° 


THE    NERVOUS   SYSTEM 


uncinate  and  inferior  longitudinal  fascicles  are  formed  by  the  shorter  and  more 
superficial  libers  of  a  larger  and  longer  tract,  the  inferior  occipitofrontal  fasciculus, 


Superior  longitudinal  fasciculus 


Uncinate  ft 


Inferior  occipitofrontal  fasciculus 

Fig.  228. — Some  of  the  long  association  bundles  projected  upon  the  lateral  aspect  of  the  cerebral 

hemisphere. 

which  unites  the  cortex  of  the  frontal  and  occipital  lobes  (Figs.  227,  228).  Along 
with  the  uncinate  fasciculus  it  may  easily  be  exposed  by  dissection,  as  it  courses 
along  the  ventrolateral  border  of  the  lentiform  nucleus. 


Cingulum 
Fas.  occipilofrontalis  sup. 

Corpus  call os um  — 
Fas.  longitudinalis  sup. 

Caudate  nucleus — sUfJ 
Internal  capsule 

Lentiform  nucleus-^ 
Insula" 
Fas.  occipitofrontalis  inf.-'' 
Fas.  uncinatus 
Amygdaloid  nucleus 

Fig.  229. — Frontal  section  of  the  cerebral  hemisphere  through  the  anterior  commissure  showing  the 
location  of  the  long  association  bundles. 

The  superior  longitudinal  fasciculus  (fasciculus  arcuatus)  is  a  bundle  of  as- 
sociation fibers  which  serves  to  connect  many  parts  of  the  cortex  on  the  lateral 


Till  CORTEX  AND  MEDULLARY  CENTEB  OF  THE  CEREBRAL  HEMISPHERE   301 

surface  of  the  hemisphere  (Tig.  228).  It  sweeps  over  the  insula,  occupying  the 
base  of  the  frontal  and  parietal  opercula,  and  then  bends  downward  into  the 
temporal  lobe  (Fig.  174).  Tt  is  composed  for  the  most  part  of  bundles  of  rather 
short  fibers  which  radiate  from  it  to  the  frontal,  parietal,  occipital,  and  temporal 

cortex. 

The  superior  occipitofrontal  fasciculus  runs  in  an  arched  course  close  to  the 

dorsal  border  of  the  caudate  nucleus  and  just  beneath  the  corpu>  callosum.  It 
is  separated  from  the  superior  longitudinal  fasciculus  by  the  corona  radiata 
(Fig.  229). 

The  weight  of  the  brain  varies  with  the  sex,  age,  and  size  of  the  individual. 
The  average  weight  of  the  brain  in  young  adult  men  of  medium  stature  is 
1360  grams.  It  is  less  in  women  and  in  persons  of  small  size  or  advanced  age. 
It  is  doubtful  if  there  is  any  close  correlation  between  the  brain  weight  and 
intelligence  or  between  the  latter  and  the  size  and  arrangement  of  the  cerebral 
convolutions  (Donaldson,  1898). 


CHAPTER  XIX 

THE  GREAT  AFFERENT  SYSTEMS 
EXTEROCEPTIVE  PATHWAYS  TO  THE  CEREBRAL  CORTEX 

As  has  been  intimated  elsewhere,  it  is  chiefly  those  nervous  impulses,  which 
are  aroused  by  stimuli  acting  upon  the  body  from  without,  that  rise  above  the 
subconscious  level  and  produce  clear-cut  sensations.  The  importance  of  these 
sensations  in  our  conscious  experience  is  no  doubt  correlated  with  the  fact  that 
it  is  through  the  reactions,  called  forth  by  such  external  stimuli;  that  the  organism 
is  enabled  to  respond  appropriately  to  the  various  situations  in  its  constantly 
changing  environment.  To  meet  these  complex  and  variable  situations  cor- 
rectly requires  the  nicest  correlation  of  sensory  impulses  from  the  various  sources 
as  well  as  their  integration  with  vestiges  of  past  experience,  and  it  is  in  connec- 
tion with  these  higher  correlations  and  adjustments  that  consciousness  appears. 
The  responses  initiated  by  interoceptive  and  proprioceptive  afferent  impulses 
are  more  stereotyped  and  invariable  in  character;  and  these  reactions  are  for  the 
most  part  carried  out  without  the  individual  being  aware  either  of  the  stimulus 
or  the  response. 

It  is  known  that  the  cerebral  cortex  is  the  organ  within  which  occur  at  least 
the  majority  of  those  complex  and  highly  variable  correlations  and  integrations 
which  have  consciousness  as  their  counterpart.  A  single  object  may  appeal 
to  many  sense  organs,  and  our  perception  of  that  object  involves  a  synthesis  of 
a  corresponding  number  of  sensations  and  their  comparison  with  past  experience. 
For  example,  when  I  meet  a  friend  and  grasp  his  hand  in  greeting,  my  perception 
of  him  includes  not  only  the  image  of  his  face  but  also  the  sound  of  his  voice 
and  the  warm  contact  of  his  hand.  Thus  thermal,  tactile,  auditory,  and  visual 
sensations  may  be  fused  in  the  perception  of  a  single  object,  and  this  involves  an 
integration  of  the  corresponding  afferent  impulses  within  the  cerebral  cortex. 
Accordingly,  it  becomes  of  special  interest  to  trace  the  course  of  these  afferent 
impulses  from  the  various  exteroceptive  sense  organs  to  their  cortical  receptive 
centers. 

As  we  shall  see,  the  outer  world  has  for  the  most  part  a  crossed  representation 

in  the  cerebral  cortex.     Cutaneous  stimuli,  received  from  objects  touching  the 
302 


nil-    i.kl.Al    .\l  l  l  ki.m    SYSTEMS  ^0. 

Hght  side  of  the  body,  and  optic  stimuli  produced  by  light  waves  coming  from 
the  right  half  of  the  field  of  vision,  are  propagated  to  the  cortex  of  the  left  hemi- 
sphere. The  crossed  representation  in  the  case  of  hearing  is  less  i  omplete,  partly 
because  every  sound  wave  reaches  both  ears,  but  also  because  the  crossing  of 
the  central  auditory  pathway  seems  to  be  incomplete. 

The  grouping  of  the  afferent  fibers  in  the  peripheral  nerves  differs  from  that 
in  the  spinal  cord.  In  each  of  the  spinal  nerves  several  varieties  of  sensory  liber- 
are  freely  mingled.  In  the  cutaneous  branches  are  found  conductors  of  thermal, 
tactile,  and  painful  sensibility;  while  the  deeper  nerves  contain  liber-  for  pain 
and  sensations  of  pre>sure-touch  as  well  as  for  muscle,  joint,  and  tendon  sensi- 
bility. Because  of  the  mtermingling  of  the  variou>  kind-  of  fibers  a  lesion  of  a 
spinal  nerve  results  in  a  loss  of  all  modalities  of  sensation  in  the  area  supplied 
exclusively  by  that  nerve. 

But  in  the  spinal  cord  a  regrouping  of  the  afferent  impulse  occurs,  such  that 
all  of  a  given  modality  travel  in  a  path  by  themselves.  All  those  of  touch  and 
pressure,  whether  originally  conveyed  by  the  superficial  or  deep  nerves,  find 
their  way  into  a  common  path  in  the  cord.  In  the  same  way  all  painful  impulses, 
whether  arising  in  the  skin  or  deeper  parts,  follow  a  special  course  through  the 
cord.  Another  intramedullary  path  conveys  impulses  from  the  muscles,  joints, 
and  tendons.  These  various  lines  of  conduction  within  the  cord  are  so  distinct 
from  each  other  that  a  localized  spinal  lesion  may  interrupt  one  without  affecting 
the  others.  A  striking  illustration  of  this  is  the  loss  of  sensibility  to  pain  and 
temperature  over  part  of  the  body  surface  without  any  impairment  of  tactile 
sensibility  as  a  result  of  a  disease  of  the  spinal  cord,  known  as  syringomyelia. 

While  we  shall  here  confine  our  attention  to  the  afferent  channels  leading 
directly  toward  the  cerebral  cortex,  it  should  not  be  forgotten  that  these  are  in 
communication  with  the  reflex  apparatus  of  all  levels  of  the  spinal  cord  and  brain 
stem. 

The  Spinal  Path  for  Sensations  of  Touch  and  Pressure. — Tactile  impulses 
which  reach  the  central  nervous  system  by  way  of  the  spinal  nerves  are  relayed 
to  the  cerebral  cortex  by  a  series  of  at  least  three  units. 

Neuron  I.— The  first  neuron  of  this  conduction  system  has  its  cell  body, 
which  typically  is  unipolar,  located  in  the  spinal  ganglion;  and  its  axon  divides 
in  the  manner  of  a  T  or  Y  into  a  central  and  a  peripheral  branch.  The  per- 
ipheral branch  runs  through  the  corresponding  spinal  nerve  to  the  skin,  or  in 
the  case  of  those  fibers  subserving  the  tactile  functions  of  deep  sensibility,  to  the 
underlying  tissues.     The  central  branch  from  the  stem  process  of  the  spinal 


3°4 


Tin:   \Kkvors  systkm 


ganglion  cell  enters  the  spinal  cord  by  way  of  the  dorsal  roots.  The  touch  fibers 
arc  probably  myelinated  and  enter  the  cuneate  fasciculus  through  the  medial 
division  of  the  dorsal  root;  and,  like  all  other  dorsal  root  libers,  they  divide  into 
ascending  and  descending  branches.  The  ascending  branches  run  for  varying 
distances  in  the  posterior  funiculus,  giving  off  collaterals  before  they  terminate 

Internal  capsule 


Thalamus 


Spinothalamic  tract 


Medial  lemniscus  -■ 


Ascending  branches  of 
dorsal  root  fibers 


Ventral  spinothalamic  tract  — 


Mesencephalon 


Medulla  oblongata 


Spinal  cord 


Dorsal  root  and  spinal  ganglion 
Fig.  230. — Diagram  of  the  tactile  path. 

m  the  gray  matter  of  the  spinal  cord,  some  few  at  least  even  reaching  the  nucleus 
gracilis  and  cuneatus  in  the  medulla  oblongata.  At  varying  levels  they  enter 
the  gray  substance  of  the  columna  posterior  and  form  synapses  with  tht 
of  the  second  order  (Fig.  230). 


le  neurons 


Tin:   GREAT    \i  i  i  ki  \  i    SYST]  305 

Neuron  II,  with  its  cell  body  located  in  the  posterior  gra\  column,  sends  ii 

axon  across  tin-  median  plane  into  the  ventral  spinothalamic  tra<  t  in  the  oppo  ite 
anterior  funiculus.  In  this  the  liber  ascends  through  the  spinal  cord  and  brain 
stem  to  the  thalamus.  This  trait  gives  oil  fibers,  either  collateral  or  terminal, 
to  the  reticular  formation  of  the  brain  stem.  Other  neurons  of  the  second  order 
in  the  tactile  path  are  located  in  the  gracile  and  euneate  nu<  lei  of  the  medulla 
oblongata,  and  their  axons  after  crossing  the  median  plane  ascend  in  the  median 
lemniscus  of  the  opposite  side  to  end  in  the  thalamus.  All  of  these  secondary 
tactile  libers  end  within  the  ventral  part  of  the  lateral  thalamic  nucleus. 

The  course  of  the  ventral  spinothalamic  tract  through  the  medulla  oblongata  anil  pons 
is  not  accurately  known.  It  has  generally  been  figured  as  joining  the  lateral  spinothalamic 
tract  dorsolateral  to  the  olive  (Fig.  230.  See  also  Herrick,  Fig.  81).  But,  since  lesions  in 
the  lateral  area  of  the  medulla  oblongata  may  cause  a  loss  of  pain  and  temperature  sensation 
over  the  opposite  half  of  the  body  without  affecting  tactile  sensibility,  it  is  not  improhahle 
that  DejVrinc  (1<)14)  is  correct  in  supposing  that  it  follows  a  median  course,  its  fibers  inter- 
mingled with  those  of  the  tectospinal  tract  which  run,  however,  in  the  opposite  direction 
(Fig.  234;  Economo,  1911;  Spiller,  1915). 

There  is  reason  to  believe  that  the  ventral  as  well  as  the  lateral  spinothalamic  tract 
consists  in  part  of  short  relays  with  synaptic  interruptions  in  the  gray  matter  of  the  spinal 
cord  and  brain  stem,  and  the  two  tracts  are  sometimes  designated  as  the  spino-reticulo-thala- 
mic  path. 

In  the  spinal  cord  there  appear  to  be  two  tracts  which  convey  tactile  im- 
pulses toward  the  brain,  an  uncrossed  one  in  the  posterior  funiculus  and  another 
that  crosses  into  the  opposite  anterior  funiculus.  Since  these  overlap  each 
other  for  many  segments,  this  arrangement  would  account  for  the  fact  that  con- 
tact sensibility  is  usually  unaffected  by  a  purely  unilateral  lesion  (Head  and 
Thompson,  1906;  Rothmann,  1906;  Petren,  1902).  Among  the  fibers  of  contact 
sensibility,  which  ascend  in  the  posterior  funiculus  to  the  euneate  and  gracile 
nuclei  of  the  same  side,  are  those  that  subserve  the  function  of  tactile  discrim- 
ination, or,  in  other  words,  the  ability  to  recognize  the  duality  of  two  closely 
juxtaposed  points  of  contact,  as  when  the  two  points  of  the  compasses  or  dividers 
are  applied  simultaneously  to  the  skin.  Furthermore,  those  elements  of  tactile 
sensibility,  which  underlie  the  appreciation  of  the  form  of  objects  or  stereognosis, 
ascend  uncrossed  in  the  posterior  funiculus  to  the  gracile  and  euneate  nuclei. 

Neuron  III. — The  neurons  located  in  the  ventral  portion  of  the  lateral  nucleus 
of  the  thalamus,  with  which  the  tactile  fibers  of  the  second  order  enter  into  syn- 
aptic relations,  send  their  axons  by  way  of  the  thalamic  radiation  through  the 
posterior  limb  of  the  internal  capsule  and  the  corona  radiata  to  the  somesthetic 
area  of  the  cerebral  cortex  in  the  posterior  central  gyrus  (Fig.  220). 


3°6 


THE   NERVOUS    SYSTEM 


THE  SPINAL  PATH  FOR  PAIN  AND  TEMPERATURE  SENSATIONS 

Tain  and  temperature  sensations  are  mediated  by  closely  associated  though 
not  identical  paths,  and  it  is  convenient  to  consider  them  at  the  same  time. 

Neuron  I. — The  first  neuron  of  this  system  has  its  cell  of  origin  located  in 
the  spinal  ganglion.     Its  axon  divides  into  a  peripheral  branch,  directed  through 

Internal  capsule 


Thalamus 


Mesencephalon 


Medulla  oblongata 


Lateral  spinothalamic  tract 


Spinal  cord 


Dorsal  root  and  spinal  ganglion 


Fig.  231. — Diagram  of  the  path  for  pain  and  temperature  sensations. 

the  peripheral  nerve  to  the  skin,  or  in  the  case  of  the  pain  fibers  also  to  the  deeper 
tissues,  and  a  central  branch,  which  enters  the  spinal  cord  through  the  dorsal 
root  and  almost  at  once  terminates  in  the  gray  matter  of  the  posterior  gray  column 
(Fig.  231).     As  was  shown  in  Chapter  VII,  there  is  reason  to  believe  that  the 


[HE    GREAT   AITKKINl    SYSTEMS  307 

libers  of  painful  sensibility,  and   possibly  those  of  temperature  sensation 
well,  are  unmyelinated  and  enter  the  cud  through  the  lateral  division  of  the 
dorsal  root  to  end  in  the  substantia  gelatinosa  Rolandi. 

Neuron  II.-  From  these  dorsal  root  fibers  the  Impulses  arc  transmitted 
(perhaps  through  the  intermediation  of  one  or  more  intercalated  neurons)  to  the 
leurons  of  the  second  order.  These  have  their  cell  bodies  Located  in  the  pos- 
terior gray  column,  and  their  axons  cross  the  median  plane  and  ascend  in  the 
lateral  spinothalamic  tract  to  end  in  the  ventral  part  of  the  lateral  nucleus  of 
the  thalamus.  In  addition  to  this  long  uninterrupted  path,  there  probably 
also  exists  a  chain  of  short  neurons  with  frequent  interruptions  in  the  gray 
matter  of  the  spinal  cord,  which  serves  as  an  accessory  path  to  the  same  end 
station.  In  the  medulla  oblongata  the  spinothalamic  tract  lies  dorsolateral  to 
the  inferior  olivary  nucleus.  In  the  pons  it  joins  the  medial  lemniscus  and 
runs  in  the  lateral  part  of  this  fillet  through  the  pons  and  mesencephalon  to  the 
thalamus  (Figs.  231,  234). 

Neuron  III.— Fibers,  arising  from  nerve-cells  located  in  the  lateral  thalamic 
nucleus,  convey  thermal  and  possibly  also  painful  impulses  to  the  somesthetic 
area  of  the  cerebral  cortex  in  the  posterior  central  gyrus  by  way  of  the  thalamic 
radiation,  and  the  posterior  limb  of  the  interal  capsule.  It  is  important  to 
note  that  it  is  not  necessary  for  painful  afferent  impulses  to  reach  the  cerebral 
cortex  before  they  make  themselves  felt  in  consciousness,  the  thalamus  being 
in  itself  sufficient  for  the  perception  of  pain  (Head  and  Holmes,  1911 ;  Head,  1918). 

The  Exteroceptive  Paths  Associated  with  the  Trigeminal  Nerve.— The  tri- 
geminal nerve  mediates  tactile,  thermal,  and  painful  sensations  from  a  large  part 
of  the  cutaneous  and  mucous  surfaces  of  the  head.  While  there  is  reason  to  be- 
lieve that  the  tactile  impulses  mediated  by  this  nerve  follow  a  central  course 
distinct  from  that  of  thermal  and  painful  sensibility,  we  cannot  as  yet  assign 
definite  paths  to  either  group,  and  shall  consider  the  exteroceptive  connections 

of  this  nerve  as  a  unit. 

Neuron  I.— The  axon  of  a  unipolar  cell  in  the  semilunar  ganglion  divides 
into  a  peripheral  branch,  distributed  to  the  skin  or  mucous  membrane  of  the 
head,  and  a  central  branch,  which  runs  through  the  sensory  root  (pars  major) 
of  the  trigeminal  nerve  into  the  pons.  Here  it  divides  into  a  short  ascending 
and  a  long  descending  branch.  The  former  terminates  in  the  main  sensory 
nucleus,  and  the  latter  in  the  spinal  nucleus  of  that  nerve  (Fig.  232). 

Neuron  II.— The  fibers  of  the  second  order  in  the  sensory  paths  of  the  tri- 
geminal nerve  arise  from  cells  located  in  the  main  sensory  and  the  spinal  nucleus 


3o8 


THE    NERVOUS   SYSTEM 


of  that  nerve;  and  alter  crossing  the  raphe  they  run  in  two  tracts  to  the  ventral 
part  of  the  lateral  nucleus  of  the  thalamus.  The  ventral  secondary  afferent 
path  is  located  in  the  ventral  part  of  the  reticular  formation,  close  to  the  spino- 
thalamic tract  in  the  medulla  oblongata  and  dorsal  to  the  medial  lemniscus  in 
the  pons  and  mesencephalon  (Figs.  132,  234).  The  dorsal  tract  lies  not  far 
from  the  floor  of  the  fourth  ventricle  and  the  central  gray  matter  of  the  cerebral 


'N  Thalamus 


'Medial  lemniscus 
Mesencephalon 

I 
I 

-1- Medial  lemniscus 
-Pons 

'"  Dorsal  secondary  tract  N.  V 
j Ventral  secondary  tract  N.  V 

Main  sensory  nucleus  N.  V 
—  Pons 

N.  V 


Spinal  tract  N.  V 

I Spinal  nucleus  N.  V 

-*■    Medulla  oblongata 
Fig.  232. — Diagram  of  the  exteroceptive  pathways  associated  with  the  trigeminal  nerve. 

aqueduct.     It  consists  in  considerable  part  of  uncrossed  fibers  and  of  fibers  hav- 
ing a  short  course  (Wallenberg,  1905;  Economo,  1911;  Dejerine,  1914). 

Neuron  III. — The  afferent  impulses  are  relayed  from  the  thalamus  to  the 
cortex  of  the  posterior  central  gyrus  by  fibers  of  the  third  order,  which  run  through 
the  posterior  limb  of  the  internal  capsule.  Their  cells  of  origin  are  located  in 
the  lateral  nucleus  of  the  thalamus. 


THE    GREA1     \l  I  I  K  l  \  l    SYSTEMS 


.W) 


The  Neural  Mechanism  for  Hearing.     The  spiral  organ  of  Corti  within  the 
cochlea  is  connected  with  the  auditory  center  in  the  cerebral  cortex  l>\  a  chain 

of  three  or  more  units. 

Neuron  I.    The  bipolar  cells  of  the  spiral  ganglion  within  the  cochlea  send 

each  a  peripheral  process  to  end  in  the  spiral  organ  of  Corti.  Ku<  h  -end-  a  <  entral 
branch  to  ramify  in  the  cochlear  nuclei,  where  it  forms  synaptic  connections 
with  the  auditor)'  neurons  of  the  second  order  (Fig.  233). 

Transverse  temporal  ^yrus 

1  Auditory  radiation 

Medial  geniculate  body 
Inferior  colliculus 


-  Lateral  lemnisci 


Collaterals  to  nucleus  of 
lateral  lemniscus 


Roslral  portion  of  the  pons 


,Stri<e  medullares 


Dorsal  cochlear  nucleus 

Ventral  cochlear  nucleus 
Cochlear  nerve 
Vestibular  nerve 


Caudal  portion  of  the  pons 

Superior  olive 

Trapezoid  body 

Nucleus  of  the  trapezoid  body 

Fig.  233. — Diagram  of  the  auditory  pathway.     (Based  on  the  researches  of  Cajal  and  Kreidl.) 


Neuron  II.— The  cells  located  in  the  ventral  and  dorsal  cochlear  nuclei  give 
rise  to  fibers,  which  after  crossing  the  median  plane  form  the  lateral  lemniscus 
of  the  opposite  side.  Those  from  the  ventral  cochlear  nucleus  cross  the  pons  in 
the  trapezoid  bod}-,  giving  off  collaterals  to  the  superior  olivary  nuclei  and  the 
nuclei  of  the  corpus  trapezoideum.  and  may  be  joined  by  fibers  taking  origin  in 
these  nuclei.  Lateral  to  the  contralateral  superior  olivary  nucleus  they  turn 
abruptly  rostrad  in  the  lateral  lemniscus.  The  fibers  from  the  dorsal  cochlear 
nucleus  run  in  the  striae  medullares  of  the  fourth  ventricle,  and  then,  dipping 


310  THE   NERVOUS    SYSTEM 

into  the  reticular  formation  of  the  pons,  cross  the  median  raphe  to  join  the  trape- 
zoid body  and  enter  the  lateral  lemniscus.  While  this  tract  is  for  the  most  part 
a  crossed  one,  sonic  fibers  probably  enter  the  lateral  lemniscus  from  the  cochlear 
nuclei  of  the  same  side.  This  accounts  for  the  fact  that  it  is  very  rare  to  have 
total  deafness  in  cither  ear  resulting  from  damage  to  the  auditory  pathway 
within  the  brain.  The  fibers  of  this  fillet  give  off  collaterals  to  the  nucleus  of 
the  lateral  lemniscus,  from  which  some  additional  fibers  may  be  contributed  to 
tin-  tract,  which  finally  terminates  in  the  medial  geniculate  body  and  the  inferior 
colliculus  of  the  corpora  quadrigemina.  The  latter,  however,  serves  only  as  a 
reflex  center,  while  the  medial  geniculate  body  is  the  way  station  on  the 
auditory  path  to  the  cerebral  cortex. 

Neuron  III. — Through  synapses  in  the  medial  geniculate  body  the  auditory 
impulses  are  transferred  to  neurons  of  the  third  order,  whose  cell  bodies  are 
located  in  this  nucleus  and  whose  fibers  run  through  the  auditory  radiation 
and  the  retrolenticular  part  of  the  internal  capsule  to  the  auditory  receptive 
center  in  the  cerebral  cortex.  It  will  be  remembered  that  this  center  is  situated 
in  the  anterior  transverse  temporal  gyrus,  located  upon  the  dorsal  surface  of 
the  temporal  lobe  within  the  lateral  cerebral  fissure,  and  in  the  small  portion 
of  the  superior  temporal  convolution  with  which  that  gyrus  is  directly  continuous. 

The  Neural  Mechanism  for  Sight. — The  nervous  impulses  responsible  for 
vision  travel  over  a  conduction  system  composed  of  at  least  four  units.  Since 
this  mechanism  has  already  been  considered  as  a  whole  on  pages  225-228 
it  is  only  necessary  for  us  to  enumerate  here  the  separate  units  of  which  it  is 
composed  (Figs.  160,  162). 

Neuron  I. — Visual  cells  of  the  retina  including  the  rods  and  cones,  which  are 
differentiated  as  receptors  for  photic  stimuli. 

Neuron  II. — Bipolar  cells  of  the  retina,  forming  synapses  with  the  visual 
cells,  on  the  one  hand,  and  the  ganglion  cells  on  the  other. 

Neuron  III.— Ganglion  cells  of  the  retina,  whose  axons  enter  the  optic  nerve, 
undergo  a  partial  decussation  in  the  optic  chiasma,  and  end  in  the  lateral  genic- 
ulate body,  pulvinar  of  the  thalamus,  and  superior  colliculus  of  the  corpora 
quadrigemina. 

Neuron  IV. — From  cells  in  the  lateral  geniculate  body  and  the  pulvinar  of 
the  thalamus  axons  run  by  way  of  the  optic  radiation  through  the  retrolenticular 
part  of  the  internal  capsule  to  the  visual  receptive  center  in  the  cerebral  hemi- 
sphere. This  is  located  in  the  cortex  on  both  sides  of  the  calcarine  fissure  and 
occupies  portions  of  the  cuneus  and  the  lingual  gyrus. 


I  in     i. KM   \  i     UFFERENT  SYSTEMS  i  i 

PROPRIOCEPTIVE  PATHWAYS 

We  have  traced  the  course  of  the  afferent  impulses  from  the  ^kin  and  from 
the  eye  and  car  to  the  cerebral  cortex,  and  have  learned  that  they  play  an  ea 
peciaUy  important  part  in  conscious  experience.  The  stimulation  of  these  ex- 
teroceptive sense  organs  initiates  both  conscious  and  reflex  adjustments  of  the 
body  to  its  environment.  But  the  resulting  movements  serve  to  excite  the 
sensory  nerve  ending  in  the  muscles,  joints,  and  tendons;  and  any  quick  move- 
ment or  change  in  position  of  the  head  will  also  excite  the  nerve  terminals  in 
the  semicircular  canals  of  the  ear.  From  these  sources  afferent  impulses  pour 
hack  into  the  nervous  system  along  special  paths  to  centers  which  to  a  great 
extent  are  separate  from  those  devoted  to  the  exteroceptive  functions  and  serve 
to  regulate  the  movements  already  initiated.  The  necessity  for  such  regulation 
is  well  illustrated  by  the  ataxic  gait  of  a  tabetic  in  whom  the  afferent  impulses 
from  the  muscles,  joints,  and  tendons  are  more  or  less  completely  lost.  In  a 
sense  the  proprioceptive  functions  of  the  nervous  system  are  secondary  to  the 
exteroceptive,  since  the  purpose  of  both  is  the  proper  adjustment  of  the  organism 
to  its  environment  by  means  of  reactions,  called  forth  by  external  stimuli, 
but  regulated  and  controlled  through  afferent  impulses  arising  within  the 
body. 

Since  in  the  regulation  of  movement  the  proprioceptive  subdivision  of  the 
nervous  system  has  to  deal  with  constant  factors,  inherent  in  the  arrangement 
of  the  muscles,  the  resultant  responses  are  more  stereotyped  and  invariable  in 
character  and  are,  for  the  most  part,  subconsciously  executed.  These  reactions 
belong  more  to  the  province  of  the  cerebellum  than  to  that  of  the  cerebrum. 

Of  the  long  ascending  channels  mediating  afferent  impulses  from  the  muscles, 
joints,  and  tendons,  only  one  extends  to  the  cerebral  cortex  by  way  of  the  thala- 
mus; all  the  others  end  in  the  cerebellum.  In  fact,  the  cerebellum  is  the  great 
correlation  center  for  afferent  impulses  of  the  propriceptive  group,  whether  they 
are  conveyed  by  the  vestibular  nerve  or  the  muscular  branches  of  the  spinal 
nerves. 

It  will  be  understood  that  on  the  motor  side  these  two  subdivisions  of  the 
nervous  system  are  not  as  distinct  as  on  the  afferent  side.  On  the  contrary, 
both  tend  to  discharge  into  common  efferent  systems.  This  is  particularly  true 
of  the  primary  somatic  motor  neuron,  which  serves  as  "the  final  common  path" 
for  both. 

The  Spinal  Proprioceptive  Path  to  the  Cerebral  Cortex.— The  conduction 
system,  along  which  those  afferent  impulses  travel  which  underlie  the  rather 


312 


THE    NERVOUS    SYSTEM 


Fig.  234.— Diagrams  showing  the  location  of  the  most  important  tracts  of  the  brain  stem 
based  on  figures  by  Dejerine.  Solid  red,  aberrant  bundles  of  the  corticobulbar  tract;  red  stipple, 
corticospinal  tract;  solid  blue,  secondary  afferent  paths  of  the  trigeminal  nerve;  horizontal  blue 
lines,  the  medial  lemniscus  (proprioceptive);  blue  stipple,  ventral  spinothalamic  tract  (or  tactile 
path);  blue  circles,  spinal  root  of  the  trigeminal  nerve;  solid  black,  lateral  spinothalamic  tract 
(pain  and  temperature);  black  triangles,  ventral  spinocerebellar  tract;  black  circles,  dorsal  spino- 
cerebellar tract;  black  stipple,  rubrospinal  tract.  A,  Through  the  mesencephalon  at  the  level  of 
the  inferior  colliculus;  B,  through  the  rostral  part  of  the  pons;  C,  through  the  medulla  at  the  level 
of  the  olive. 


IMI     GR]   \  I     AFFERENT    SYST]  MS 


313 


vague  sensations  of  position  and  posture  and  of  active  and  passive  movements, 
consists  of  a  chain  of  at  least  three  units. 

Neuron  I.-  The  cell  bodies  of  the  neurons  of  the  first  order  belonging  to  this 
system  are  loeated  in  the  spinal  ganglia.  Their  axons  are  myelinated  and  divide 
into  peripheral  branches,  running  to  speeiali/.ed  end  organs  within  the  muscles, 
joint-  and  tendons,  and  central  branches  directed  through  the  medial  division 
of  the  dorsal  root  into  the  posterior  funiculus  of  the  spinal  cord.  Here  the) 
divide;  and  their  ascending  branches  run  through  the  posterior  funiculus  to 
terminate  in  the  gracile  and  cuneate  nuclei  of  the  medulla  oblongata,  where  they 
enter  into  synaptic  relations  with  neurons  of  the  second  order  (Fig.  235). 

Neuron  II. — From  cells  located  in  the  gracile  and  cuneate  nuclei  the  axons 
run  as  internal  arcuate  fibers  across  the  median  raphe  in  the  medulla  oblongata 
and  ascend  by  way  of  the  medial  lemniscus  to  end  in  the  ventral  part  of  the  lateral 
nucleus  of  the  thalamus,  where  they  form  synapses  with  neurons  of  the  third  order. 

Neuron  III.- — From  cells  in  the  lateral  nucleus  of  the  thalamus  fibers  pass  by 
way  of  the  thalamic  radiation  through  the  posterior  limb  of  the  internal  capsule 
to  the  posterior  central  gyrus  or  somesthetic  area  of  the  cerebral  cortex. 

SPINAL  PROPRIOCEPTIVE  PATHS  TO  THE  CEREBELLUM 

Impulses  from  the  muscles,  joints,  and  tendons  may  reach  the  cerebellum  by 
three  routes: 

A.  By  Way  of  the  Dorsal  External  Arcuate  Fibers: 

Neuron  I  of  this  chain  is  the  same  as  in  the  path  to  the  cerebral  cortex  just 
described,  the  fibers  from  the  dorsal  root  reaching  the  gracile  and  cuneate  nuclei. 

Neuron  II. — From  cells  located  in  these  nuclei  axons  run  as  posterior  external 
arcuate  fibers  to  the  restiform  body  of  the  same  side,  and  thence  through  the 
white  center  of  the  cerebellum,  to  end  in  the  cerebellar  cortex  (Fig.  235,  red). 

B.  By  Way  of  the  Ventral  Spinocerebellar  Tract: 

Neuron  I. — The  first  neuron  in  this  chain  is  similar  to  the  primary  neuron  in 
the  two  preceding  paths.  The  impulses,  however,  travel  over  collateral  and 
terminal  branches  of  the  dorsal  root  fibers  to  reach  the  posterior  gray  column 
and  intermediate  gray  matter  of  the  spinal  cord. 

Neuron  II. — From  cells  located  in  the  posterior  gray  column  and  intermediate 
gray  matter  fibers  run  in  the  ventral  spinocerebellar  tracts  of  the  same  or 
opposite  side  through  the  spinal  cord,  medulla  oblongata  and  pons,  bend  around 
the  brachium  conjunctivum,  and  then  course  back  along  the  anterior  medullary 
velum  to  the  cortex  of  the  rostral  part  of  the  vermis  (Fig.  235,  blue). 


3H 


THE    NERVOUS    SYSTEM 


('.By  Way  of  the  Dorsal  Spinocerebellar  Tract: 

Neuron  I.— The  first  neuron  of  this  chain  is  similar  to  the  primary  neuron 
in  the  three  preceding  paths.  The  impulses,  however,  travel  over  those  col- 
lateral and  terminal  branches  of  the  dorsal  root  fibers  which  ramify  about  the 
cells  of  the  nucleus  dorsalis. 

Internal  capsule 


Thalamus 


.■  I  'eniral  spinocerebellar  tract 


Cerebellum 


Dorsal  external  arcuate  fiber 


• Ventral  spinocerebellar  tract 


Ascending  branches  of 
dorsal  rool  fibers 


Dorsal  spinocerebellar  tract 
^f^S^  Dorsal  root  and  spinal  ganglion 


Fig.  235. — The  proprioceptive  paths. 

Neuron  IT— From  cells  in  the  nucleus  dorsalis  fibers  run  to  the  dorsal  spino- 
cerebellar tract  of  the  same  side  and  through  the  restiform  body  to  the  cortex 
of  both  the  rostral  and  the  caudal  portions  of  the  vermis  (Fig.  235,  red). 

Cerebellar   Connections   of   the   Vestibular   Nerve.— The   vestibular   nerve 


THE    GREAT    AFFERENT    SYST1  M  5 


315 


conducts  impulses  from  specialized  sense  organs  in  the  semicircular  canals 
cule  and  utricle,  which  are  stimulated  by  movements  and  changes  in  posture 

of  the  head. 

Neuron  I.— From  the  bipolar  cells  of  the  vestibular  ganglion  (of  Scarpa 
located  within  the  internal  auditory  meatus,  peripheral  processes  run  to  the 
macula*  of  the  utricle  and  saccule  and  to  the  crista;  of  the  semicircular  canals. 
The  central  processes  are  directed  through  the  vestibular  nerve  toward  the 
floor  of  the  fourth  ventricle  and  divide  into  ascending  and  descending  branches 
While  the  descending  and  many  of  the  ascending  branches  terminate  in  the 
vestibular  nuclei,  many  other  ascending  branches  pass  without  interruption  to 
end  in  the  cerebellar  cortex  and  particularly  in  that  of  the  vermis  (Fig.  136). 

Neuron  II. — Some  of  the  cells  situated  in  the  vestibular  nuclei  send  their 
axons,  along  with  the  ascending  branches  mentioned  above  in  the  vestibulo- 
cerebellar tract,  to  the  cortex  of  the  vermis,  and  to  a  less  extent  to  the  cortex 
of  the  cerebellar  hemispheres  also. 


CHAPTER  XX 

EFFERENT  PATHS  AND  REFLEX  ARCS 

The  motor  apparatus  is  a  complex  mechanism  into  which  the  pyramidal 
system  enters  as  a  single  factor.  The  primary  motor  neurons  of  the  brain  stem 
and  spinal  cord  are  also  under  the  influence  of  other  motor  centers  than  those 
found  in  the  cerebral  cortex.  They  receive  impulses  from  the  corpora  quadri- 
gemina  through  the  tectospinal  tract,  from  the  lateral  vestibular  nucleus  by  way 
of  the  vestibulospinal  tract,  from  the  large  motor  cells  of  the  reticular  formation 
through  the  reticulospinal  path,  from  the  cerebellum,  and  probably  also  from 
the  corpus  striatum  by  way  of  the  red  nucleus  and  the  rubrospinal  fasciculus. 
Perhaps,  also,  impulses  descend  from  the  thalamus  or  subthalamus  by  way  of  a 
thalamospinal  tract. 

We  must  not  think  of  the  individual  parts  of  this  complex  mechanism  as 
functioning  separately,  since  each  of  these  motor  centers  contributes  its  share 
to  the  control  of  the  primary  motor  neuron,  upon  which  as  the  "final  common 
path"  all  these  efferent  pathways  converge.  Only  by  keeping  this  fact  con- 
stantly in  mind  can  the  motor  functions  be  properly  understood.  The  same 
idea  has  been  well  stated  by  Walshe  (1919):  'Tn  stimulation  experiments  on  the 
motor  cortex  we  see  a  complex  motor  mechanism  at  work  under  the  influence 
of  an  abnormally  induced,  crude  form  of  hyperactivity  of  the  predominant  partner 
in  this  mechanism.  Conversely,  after  destructive  lesions,  we  observe  it  at  work 
liberated  from  the  control  of  this  predominant  partner  and  deprived  of  its  actual 
cooperation." 

On  the  other  hand,  the  grave  motor  disturbances  resulting  from  lesions  in 
the  basal  ganglia  and  especially  the  corpus  striatum  with  little  or  no  involvement 
of  the  corticospinal  tracts  (paralysis  agitans,  Auer  and  McCough,  1916;  bilateral 
athetosis,  Cecile  Vogt,  1911;  and  progressive  lenticular  degeneration,  Wilson, 
1912-14)  have  recently  called  attention  to  the  importance  of  the  corpus  striatum 
and  the  extrapyramidal  motor  path  (see  p.  324).  In  these  diseases  voluntary 
movements  are  impeded  by  tremor,  rigidity,  and  athetosis;  and  in  all  probability 
these  disturbances  arise  because  the  pyramidal  system  is  deprived  of  the  co- 
operation of  one  of  the  subordinate  "partners"  in  the  motor  combine. 
316 


EFFERENT  PATHS  AND  REFLEX  ARCS 


317 


Even  after  cerebral  control  has  been  entirely  eliminated  in  the  dog  by  de- 
cerebration,  many  reflex  functions  remain,  which  represent  the  unguided  activity 
of  the  lower  elements  in  the  motor  mechanism;  and  we  now  know  that  a  similar 
independent  reflex  activity  may  occur  in  the  spinal  cord  of  man  after  total  trans- 
verse lesions  (Riddoch,  1917). 

THE  GREAT  MOTOR  PATH 

The  great  motor  path  from  the  cerebral  cortex  to  the  skeletal  musculature, 
through  which  the  bodily  activities  are  placed  directly  under  voluntary  control, 
is  in  man  and  mammals  the  dominant  factor  in  the  motor  mechanism.  We 
have  seen  that  afferent  channels  from  the  various  exteroceptors  reach  the  cere- 


Fig.  236. — Cortical  localization  upon  the  lateral  aspect  of  the  human  cerebral  hemisphere.    (Starr.) 

bral  cortex;  and  that  through  the  correlation  of  the  olfactory,  auditory,  visual, 
tactile,  thermal,  and  painful  afferent  impulses  which  pour  into  it,  there  is  built 
up  within  the  cortex  a  representation  of  the  outer  world  and  its  constantly  chang- 
ing conditions.  The  responses  appropriate  to  meet  the  entire  situation  in  which 
the  individual  finds  himself  from  moment  to  moment  are  in  large  part  at  least 
initiated  in  the  cerebral  cortex  and  are  executed  through  the  motor  mechanism. 
In  these  responses  the  great  motor  path  is  the  dominant  factor,  although  other 
parts  of  the  mechanism  are  secondarily  called  into  action,  especially  the  pro- 
prioceptive reflex  arcs,  including  the  coordinating  and  tonic  mechanism  of  the 
cerebellum. 

This  great  motor  path  consists  of  two-unit  chains.     The  so-called   upper 
motor  neurons  conduct  impulses  from  the  motor  cortex  to  the  motor  nuclei  of  the 


3*8 


THE    NERVOUS    SYSTEM 


cerebral  nerves  or  to  the  anterior  gray  columns  of  the  spinal  cord;  whence  the 
lower  molar  neurons,  also  known  as  primary  motor  neurons,  relay  the  impulses 
to  the  muscles.  It  is  possible  that  another  and  much  shorter  element  is  inter- 
calated between  the  two  chief  units  of  this  conduction  system. 

The  motor  cortex  occupies  the  rostral  lip  of  the  central  sulcus  and  the  ad- 
jacent portion  of  the  anterior  central  gyrus,  extending  over  the  dorsal  border  of 


Motor  cortex 


Posterior  limb  of  internal  capsule 


..Genu  of  corpus  callosum 


Basis  pcdunculi  of  mesencephalon 


--^Longitudinal  fascicles  of  pons 


— Pyramid  of  medulla  oblongata 

,11k--— —Lateral  corticospinal  tract 

^J|y*^ Ventral  corticospinal  tract 

Fig.  237. — The  corticospinal  path. 

the  hemisphere  into  the  paracentral  lobule.  Within  this  area  the  skeletal  mus- 
culature is  represented  in  inverted  order,  that  moving  the  toes  near  the  dorsal 
border  of  the  hemisphere.  The  area  from  which  the  corticobulbar  tract  arises 
is  only  a  small  part  of  the  whole,  and  is  situated  near  the  lateral  cerebral  fissure 
(the  region  marked  Eyelids,  Cheeks,  Jaws,  Lips  in  Fig.  236).  From  all  the  rest 
of  the  motor  cortex  arise  the  fibers  of  the  corticospinal  tract. 


EFFKRKNT    PATHS    AND    REFLEX    ARCS 


319 


The  motor  path  for  the  spinal  nerves  Includes  the  corticospinal  tra<  1  and  the 
spinal  primary  motor  neurons. 

Neuron  I,  or  upper  motor  neuron.  The  gianl  pyramidal  cells  of  the  motor 
cortex  give  rise  to  the  fibers  of  the  corticospinal  trad,  which  is  also  known  as 


!■  1     in  1  longitudinalis  cerebri 
Radiatio  corporis  callostx 

Septum  pellucidum 

l'lexus  cliorio- 
ideus  ventriculi  jj^fll 
lateralis         " 

Corona  radiata 


Columna, 
fornicis 
'Plexus  chorio- 
ideus  ventriculi 

tertii 
Caosula  interna 


Thalamus 

Ventriculus 

tertius 
Fossa  inter- 
peduncular 
( i'arini) 

Cornu  inferius 
ventriculi 
lateralis 


Pedunculu- 
cerebri 

Brachium  pout 

Fasciculi  longitudi-' 

nales  (pyramidalcs) 

pontis 

Facies  inferior  cerebelli-' 
Fibrae  pontis  superficiales' 

Pyramis  medullae  oblongatae  ' 


,  1  .)  111;  frontalis  superior 

,  Truncui  cprporii  callosi 

_  Cornu  anli nil     v<  ntriculi 

lateralis 

Caput  nuclei  caudati 


Globus  pallidus 
Tractus  opticus 


N.  trigeminus 


Nn.  facialis  und 
acusticus 


Flocculus 
glossopliaryngcus 
J.  vagus 
^Nucleus  olivaris  inferior 
"v-»  Decussatio  pyramidum 


.  Fig.  238.— Section  through  the  brain  in  the  axis  of  the  brain  stem,  showing  the  entire  extent  of 

the  corticospinal  tract.      (Toldt.) 

the  cerebrospinal  fasciculus  or  pyramidal  tract.  These  fibers  traverse  the  rost  ral 
half  of  the  posterior  limb  of  the  internal  capsule,  the  intermediate  three-fifths 
of  the  basis  pedunculi,  the  basilar  portion  of  the  pons,  and  the  pyramid  of  the 
medulla  oblongata,  and  after  undergoing  a  partial  decussation  are  continued  into 
the  spinal  cord  (Figs.  237,  238).     At  the  pyramidal  decussation  in  the  caudal 


\20 


THE   NERVOUS    SYSTEM 


part  of  the  medulla  oblongata  the  greater  part  of  the  tract  crosses  to  the  opposite 
side  oi  the  spinal  cord  and  is  continued  as  the  lateral  corticospinal  tract  in  the 
lateral  funiculus.  The  smaller  part  is  continued  directly  into  the  ventral  fu- 
niculus of  the  same  side,  as  the  ventral  corticospinal  tract.  The  fibers  of  the 
ventral  tract  cross  the  median  plane  a  few  at  a  time  and  terminate,  as  do  those  of 
the  lateral  tract,  directly  or  indirectly  in  synaptic  relations  with  the  primary 
motor  neurons  within  the  anterior  gray  column  (Fig.  239).  The  ventral  tract 
is  not  evident  as  a  well-marked  bundle  below  the  level  of  the  midthoracic  region. 


Mesencephalon 
N.IV 

Pons 

Corticobulbar  tract 


Medulla  oblongata 

Ventral  corticospinal  tract 
Lateral  corticospinal  tract — 

Spinal  cord 


I  'cntral  root 
Fig.  239. — The  corticobulbar  and  corticospinal  tracts. 

It  has  long  been  known  that  in  the  higher  mammals  the  lateral  pyramidal  tract,  although 
consisting  predominatingly  of  crossed  fibers,  contains  a  few  homolateral  fibers  also  (Simpson, 
1902).  and  according  to  the  observations  of  Dejerine  (1914)  and  other  investigators  this 
holds  true  for  man  also.  Dejerine  speaks  of  these  uncrossed  fibers  in  the  lateral  corticospinal 
tract  as  a  third  bundle  arising  out  of  the  motor  decussation,  and  calls  it  the  ''homolateral" 
corticospinal  fasciculus.  A  good  account  of  this  tract  and  of  the  superficially  placed  bundle 
of  uncrossed  pyramidal  fibers  that  is  to  be  found  in  the  ventral  part  of  the  lateral  funiculus 
in  the  cervical  portion   of  the  spinal  cord  is  given  by  Barnes   (1901). 


Neuron  II.  —To  the  lower  or  primary  motor  neurons  belong  the  large  multi- 
polar cells  of  the  anterior  gray  column  of  the  spinal  cord.  These  give  rise  to  the 
motor  fibers  that  leave  the  spinal  cord  through  the  ventral  roots  and  are  dis- 
tributed through  the  spinal  nerves  to  the  skeletal  musculature. 

The  motor  path  for  the  cranial  nerves  is  less  well  known.  It  includes  the 
corticobulbar  tract  and  those  fibers  of  the  cranial  nerves  which  innervate  striated 
musculature. 

Neuron  I,  or  upper  motor  neuron.     The  corticobulbar  fibers  arise  from  the 


I  I  11  Rl  NT  PATHS  AND  REFLEX  ARCS 


321 


giant  pyramidal  cells  of  the  part   of  the  motor  cortex   near  the  lateral  fissure. 
These  fibers  run  through  the  genu  of  the  internal  capsule  and  the  basis  pedunculi 

to  end,  directly  or  indirectly,  in  synaptic  relation  to  the  primary  motor  neurons 
of  the  somatic  motor  and  special  visceral  motor  nuclei  of  the  brain  stem.      Be 
fore  terminating,  the  majority  cross  the  median  plane,  but  some  end  in  the  motor 
nuclei  of  the  same  side  (Fig.  239). 

Neuron  II,  lower  or  primary  motor  neuron.  From  the  large  multipolar 
cells  of  the  somatic  motor  and  special  visceral  motor  nuclei  arise  fibers,  whii  h 
run  through  the  cranial  nerves  to  end  in  striated  musculature. 


Tr.  corticosp. 
Tr.  corticobulb. 

F.  A.  Sth.  (Ill) 

F.  A.  Pd.        Tr.  cb.  lot. 
(Ill,  VI, XI)   Jr.  cb.  med. 

/•'.  A.  P.  (V,  X,  XI,  XII) 

Pons 


F.A.B.  P.  (VII) 

Tr.  corticosp. 

Tr.  corticobulb. 

XI,  C  1 1 -IV 

C  II-IV 

Tr.  corticosp.  mcd. 

C  II-IV 
XI,  C II-IV 


Pulvinar 

Med.  Ii  mniscus 
Nuc.  A  .  /// 

Corpora  quad. 

Nuc.  N.IV 
Nuc.  N.  V 

Fourth  vent. 
Nuc.  N.  VI 

Nuc.  N.  VII 

Nuc.  ambiguus  Nn.  IX  and  X 

Nuc.  N.  XII 

Xuc.  gracilis 
Nuc.  cuncatus 
Xuc.  XI 

XI,  XII,  C  II-IV 
Tr.  corticosp.  lat. 


Fig.  240. — The  course  of  the  fibers  of  the  corticobulbar  tract.  Redrawn  from  Dejerine. 
Corticobulbar  tract,  solid  black;  corticospinal  tract,  vertical  lines;  the  medial  lemniscus,  horizontal 
lines.  F.  A.  B.  P.,  Bulbopontine  aberrant  fibers;  7"'.  A.  P.,  aberrant  fibers  of  the  pons;  F.  A.  Pd., 
aberrant  fibers  of  the  peduncle;  F.  A.  Sth.,  subthalamic  aberrant  fibers;  Tr.  cb.  lat.,  tractus  cortico- 
bulbaris  lateralis;  Tr.  cb  med.,  tractus  corticobulbaris  medialis.  The  Roman  numerals  indicate 
the  nuclei  of  the  cranial  and  cervical  nerves  which  are  supplied  by  the  various  bundles. 


The  Corticobulbar  Tract. — According  to  Dejerine  (1914).  who,  because  of  the  careful 
study  which  he  and  his  associates  have  made  of  this  efferent  system,  is  most  entitled  to  speak 
authoritatively  on  the  subject,  the  corticobulbar  fibers  occupy  chiefly  the  medial  part  of  the 
basis  pedunculi  and  its  deeper  layer.  The  fibers  separate  into  two  major  groups.  One 
part  follows  the  course  of  the  corticospinal  tract  and  descends  in  the  basilar  portion  of  the 
pons  and  the  pyramids  of  the  medulla  oblongata.  Another  part,  which  he  designates  as 
the  system  of  aberrant  pyramidal  fibers,  detaches  itself  from  the  preceding  in  small  bundles 
at  successive  levels  of  the  brain  stem.  These  enter  the  reticular  formation  and  descend 
within  the  region  occupied  by  the  medial  lemniscus,  giving  off  fibers  to  the  motor  nuclei  of 
the  cranial  nerves  (Fig.  240).     The  fibers  undergo  an  incomplete  decussation  in  the  raphe 


322 


THE    NERVOUS    SYSTEM 


and  go  chiefly  to  the  nuclei  of  the  opposite  side.  The  decussating  fibers  are  grouped  in  very 
small  bundles,  those  for  a  given  nucleus  crossing  at  the  level  of  that  nucleus.  There  is  great 
variation  in  the  course  of  the  bundles  of  aberrant  pyramidal  fibers  in  different  brains. 

The  chief  aberrant  bundles  which  can  be  traced  dorsalward  into  the  reticular  formation 
(indicated  in  solid  red  in  Fig.  234)  are  as  follows: 

1.  The  aberrant  fibers  of  the  peduncle  (Fig.  240,  F.  A.  Pd.)  form  two  bundles,  which 
have  been  called  by  some  authors  the  median  and  lateral  corticobulbar  tracts.  These 
descend  in  the  territory  of  the  medial  lemniscus  (Figs.  234,  240)  and  give  off  fibers  to  the 
nuclei  of  the  third,  sixth,  and  eleventh  cranial  nerves.  With  these  two  bundles  run  some 
libers  destined  for  the  upper  cervical  segments  of  the  spinal  cord.  This  group  of  aberrant 
fibers  therefore  controls  the  movements  of  the  eyes  and  the  associated  movements  of  the  head. 

1.  The  aberrant  fibers  of  the  pons  (Fig.  240,  F.  A.  P.)  which  join  the  preceding  in  the 
medial  lemniscus  run  to  the  motor  nuclei  of  the  trigeminal  and  hypoglossal  nerves  and  to  the 
nucleus  ambiguus. 

3.  The  bulbopontine  aberrant  fibers  (Fig.  240,  F.  A.  B.  P.)  leave  the  main  trunk  of  the 
pyramidal  system  near  the  level  of  the  sulcus  between  the  pons  and  medulla.  They  reinforce 
the  preceding  groups,  supply  the  motor  nucleus  of  the  facial  nerve,  and  send  fibers  to  the 
nucleus  ambiguus  and  to  that  of  the  hypoglossal  nerve. 

These  facts  are  of  the  greatest  importance  for  the  clinical  neurologist.  Lesions  re- 
stricted to  the  basilar  portion  of  the  pons  are  likely  to  destroy  at  the  same  time  the  cortico- 
spinal fibers  and  those  of  the  corticobulbar  tract  which  end  in  the  facial  nucleus.  A  lesion 
confined  to  the  reticular  formation  and  involving  the  medial  lemniscus  may,  according  to  its 
level,  sever  the  corticobulbar  fibers  for  the  motor  nuclei  of  the  eye-muscle  nerves  or  those 
for  the  motor  nuclei  of  the  trigeminal,  accessory,  and  hypoglossal  nerves  without  involve- 
ment of  the  corticospinal  tracts.  Conjugate  deviation  of  the  head  and  eyes,  not  often  seen 
as  a  result  of  damage  to  the  basilar  portion  of  the  pons,  may  result  from  tegmental  lesions 
involving  the  aberrant  fibers  of  the  peduncle. 


The  physiologic  and  clinical  significance  of  the  course  of  the  corticospinal  and 
corticobulbar  tracts  is  obvious.  It  is  because  of  the  decussation  of  these  fibers 
that  the  muscular  contractions  produced  by  cortical  stimulation  occur  chiefly 
on  the  opposite  side  of  the  body,  and  that  the  paralyses  resulting  from  lesions 
in  the  pyramidal  system  above  the  decussation  are  contralateral.  If  the  lower 
motor  neuron  is  injured,  the  associated  muscle  atrophies  and  a  flaccid  paralysis 
results.  Injury  to  the  upper  motor  neuron,  on  the  other  hand,  leads  to  a  loss 
of  function  without  atrophy,  but  rather  with  an  increased  tonicity  of  the  affected 
muscle,  i.  e.,  to  a  spastic  paralysis.  By  means  of  such  differential  characteristics 
as  these  it  is  possible  to  tell  which  of  the  two  links  in  the  motor  chain  has  been 
broken. 

In  order  to  understand  the  combination  of  symptoms,  which  result  from 
damage  to  the  motor  path  at  different  levels,  it  is  necessary  to  have  in  mind  the 
topography  of  its  constituent  parts.  Some  of  these  relations  are  indicated  in 
Fig.  241.  Since  the  motor  cortex  is  spread  out  over  a  rather  extensive  area, 
it  is  usually  not  entirely  destroyed  by  injury  or  disease.     A  restricted  cortical 


EFFERENT  PATHS  AND  REFLEX  ARCS 


323 


lesion  may  cause  a  monoplegia,  i.  c  paralysis  of  a  single  part,  such  as  the  arm  or 
leg  (Fig.  241,  A).  But  in  the  internal  capsule  the  motor  fibers  are  grouped 
within  a  small  area  and  are  frequently  all  destroyed  together.  This  causes 
paralysis  of  the  opposite  hall"  of  the  body  or  hemiplegia  (Fig.  241,  B).  Damage 
to  the  pyramidal  system  in  the  cerebral  peduncle,  pons,  or  upper  part  of  the 
medulla  oblongata  may  also  cause  hemiplegia;  but  in  such  cases  those  cortico- 


N.  VII 


N.XII 


To  the  arm 


•To  the  leg 
Fig.  241. — Diagram  to  illustrate  the  effects  of  lesions  in  various  parts  of  the  motor  path. 


bulbar  fibers,  which  leave  the  main  strand  of  pyramidal  fibers  above  the  level 
of  the  lesion,  may  escape  injury  and  the  corresponding  cranial  nerves  need  not 
be  involved  (Fig.  241,  C).  Furthermore,  in  lesions  of  the  brain  stem  the  motor 
nucleus  or  emergent  fibers  of  one  of  the  cranial  nerves  may  be  destroyed  along 
with  the  pyramidal  fibers,  in  which  case  there  would  result  a  paralysis  of  the 
muscles  supplied  by  that  nerve  as  well  as  a  paralysis  of  the  opposite  half  of  the 
body  below  that  level — a  crossed  paralysis  (Fig.  241,  C).     While  damage  to  the 


o24  THE   NERVOUS    SYSTEM 

spinal  cord  may  affect  only  one  lateral  half  and  cause  a  homolateral  paralysis 
below  the  lesion  (Fig.  241,  D),  it  is  common  for  both  lateral  halves  to  be  involved 
and  for  the  resulting  paralysis  to  be  bilateral  (Fig.  241,  E). 

The  Extrapyramidal  Motor  Paths.— In  recent  years  it  has  become  increasingly  evi- 
dent that  the  pyramidal  system  is  not  the  only  channel  through  which  volitional  impulses 
are  able  to  reach  the  primary  motor  neurons  of  the  brain  stem  and  spinal  cord.  Rothmann 
(1907)  found  that,  after  section  of  the  lateral  corticospinal  and  the  rubrospinal  tracts  in 
monkeys  at  the  level  of  the  third  cervical  nerve,  voluntary  movements  were  lost  for  a  time, 
Inn  soon  reappeared;  and  lie  concluded  that  there  must  be  an  extrapyramidal  volitional 
path  in  the  ventral  funiculus.  Three  years  later  Schafer  (1910)  showed  that  in  monkeys 
the  paralysis,  which  results  from  section  of  the  pyramids  of  the  medulla  oblongata,  is  not 
complete  and  persistent ;  and  he  agreed  with  Rothmann  that  there  must  be  some  other  path 
for  volitional  impulses.  He  believes  that  this  alternative  path  is  formed  by  descending 
libers  in  the  ventral  funiculus  and  in  the  ventral  part  of  the  lateral  funiculus,  since  section 
of  these  fibers  produces  as  complete  and  persistent  paralysis  in  monkeys  as  does  section  of 
the  pyramids  themselves. 

Sherrington  and  Graham  Brown  (1913)  excised  the  arm  area  of  the  cerebral  cortex 
in  the  chimpanzee,  and  found  that  function  in  the  corresponding  limb  was  completely  re- 
stored in  a  few  weeks.  They  were  able  to  show  that  this  was  not  attributable  to  the  vicarious 
activity  of  the  corresponding  postcentral  or  the  opposite  precentral  cortex.  Horsley's  (1909) 
patient,  who  recovered  some  degree  of  control  over  the  arm  after  the  removal  of  its  cortical 
center  in  the  precentral  gyrus,  shows  that  the  observations  of  Sherrington  and  Brown  are  at 
least  in  part  applicable  to  man. 

We  know  that  the  cerebral  cortex  is  connected  through  efferent  projection  tracts  with 
the  thalamus  and  red  nucleus  and  through  collaterals  from  the  corticospinal  fibers  with  the 
corpus  striatum  (Cajal).  But  we  do  not  know  which,  if  any,  of  these  systems  of  projection 
fibers  constitutes  a  part  of  the  extrapyramidal  path  for  volitional  impulses. 

A  great  deal  of  attention  has  recently  been  given  by  clinical  neurologists  to  the  dis- 
turbance of  voluntary  movement  by  tremor,  rigidity,  and  athetosis,  which  results  from  lesions 
of  the  corpus  striatum.  This  body  seems  to  contain  an  important  motor  center,  and  ac- 
cording to  Wilson  (1912  and  1914)  it  exerts  a  steadying  influence  upon  voluntary  move- 
ments. The  globus  pallidus  seems  to  be  connected  with  the  spinal  primary  motor  neurons 
by  way  of  the  striorubral  and  rubrospinal  tracts.  It  is  also  possible,  especially  in  view 
of  the  important  motor  functions  attributed  to  the  ventrolateral  descendnig  tracts  of  the 
spinal  cord  by  Rothmann  and  Schafer,  that  efferent  impulses  reach  the  spinal  cord  from  the 
globus  pallidus  by  way  of  the  substantia  nigra  over  the  strionigral,  the  somewhat  hypothetic 
nigroreticular,  and  the  reticulospinal  tracts.  It  is  known  that  the  axons  arising  in  the  sub- 
stantia nigra  run  into  the  reticular  formation  of  the  mesencephalon,  beyond  which  they 
cannot  be  traced  (Cajal,  1911).  According  to  Collier  and  Buzzard  (1901)  the  rubrospinal, 
vestibulospinal,  tectospinal,  and  reticulospinal  tracts  probably  represent  the  original  paths 
for  impulses  from  higher  to  lower  parts  of  the  nervous  system;  and  the  path  from  the 
cerebrum  to  the  spinal  cord,  at  first  indirect,  has  been  short-circuited  in  the  mammal 
through  the  evolution  of  the  pyramidal  system. 

When  it  is  remembered  that  the  pyramidal  system  is  a  late  development,  present  only 
in  mammals,  it  does  not  seem  unreasonable  to  think  that  some  other  and  older  path  for 
volitional  impulses  may  also  exist.  The  globus  pallidus,  the  representative  of  the  primitive 
corpus  striatum  of  the  lower  vertebrates,  has  been  called  the  paleostriatum  (Elliot  Smith, 
1919).     From  this  basal  nucleus  there  arises  in  all  vertebrates  an  important  efferent  bundle, 


EFFERENT  PATHS  AM)  REFLEX  ARCS 


325 


"the  basal  forebrain  bundle"  of  Edinger  (1887),  which  is  represented  in  mammals  by  the 

striofugal  fibers  of  1  he  ansa  lenticularis.  Ii  is  clear  1  hat  this  fascicle,  which  persists  through- 
out the  vertebrate  ^series,  must  subserve  important  functions;  and  it  is  probable  thai  it 
forms  a  part  of  the  extrapyramidal  motor  path. 


THE  CORTICO-PONTO-CEREBELLAR  PATH 

The  cortico-ponto-cerebellar  path  is  an  important  descending  conduction 
system  which  places  the  cerebellum  under  the  influence  of  the  cerebral  cortex. 
Since  a  part  of  the  corticopontine  fibers  are  collaterals  given  off  to  the  nu<  lei 
of  the  pons  by  the  corticospinal  fibers,  and  since  in  many  mammals  practically 

Red  uitcli  us 


Purkinje  cell 

( '<  1  <hi  1 1  n  ni 


Dentate  nucleus 

'- —  Braehium  conjunctivum 

"  Braehium  pontis 
" "  Rubrospinal  tract 

—  Corticospinal  tract 


Fig.  242. — The  cortico-ponto-cerebellar  and  cerebello-rubro-spinal  paths.     (Modified  from  Cajal.) 

all  of  the  corticopontine  fibers  are  represented  by  such  collaterals  (Cajal,  1909), 
one  can  scarcely  avoid  the  conclusion  that  through  this  system  the  coordinating 
and  tonic  mechanism  of  the  cerebellum  is  brought  into  play  for  the  regulation 
of  movements  initiated  from  the  cerebral  cortex.  In  this  sense  the  idea  of 
Cajal  (1911)  that  there  exists  an  indirect  motor  path  to  the  spinal  cord  through 
the  cerebellum  is  probably  correct  (Fig.  242). 

Neuron  I.— From  pyramidal  cells  in  the  frontal  lobe  of  the  cerebral  cortex 
fibers  pass  through  the  anterior  limb  of  the  internal  capsule  and  the  medial  one- 


326  THE    NERVOUS    SYSTEM 

fifth  of  the  basis  pedunculi;  and  similar  fibers  from  the  temporal  lobe  descend 
through  the  sublenticular  part  of  the  internal  capsule  and  the  lateral  one-fifth 
of  the  basis  pedunculi.  These  fibers,  together  with  the  corticospinal  tract, 
form  the  longitudinal  fasciculi  of  the  pons;  and,  along  with  collaterals  from  that 
tract,  they  end  within  the  nuclei  pontis  in  synaptic  relations  with  the  neurons 
of  the  second  order  (Figs.  106,  242). 

Neuron  II. — Arising  from  cells  in  the  nuclei  pontis,  the  transverse  fibers  of 
the  pons  cross  the  median  plane  and  run  by  way  of  the  brachium  pontis  and 
white  substance  of  the  cerebellum  to  the  cerebellar  cortex  of  the  opposite  side. 

THE  CEREBELLO-RTJBRO-SPINAL  PATH 

The  cerebello-rubro-spinal  path  is  the  conduction  system  through  which  the 
cerebellum  contributes  its  important  share  to  the  control  of  the  primary  motor 
neurons  of  the  spinal  cord  in  the  interest  of  muscular  coordination,  equilibration, 
and  the  maintenance  of  muscle  tone.  Other  efferent  connections  of  the  cerebel- 
lum have  been  discussed  on  page  211. 

Neuron  I. — From  the  Purkinje  cells  of  the  cerebellar  cortex  fibers  run  to 
terminate  in  the  central  nuclei  of  the  cerebellum,  especially  the  dentate  nucleus 
(Fig.  242). 

Neuron  II. — Arising  chiefly,  if  not  entirely,  from  the  cells  of  the  dentate 
nucleus,  fibers  run  through  the  brachium  conjunctivum,  undergo  decussation 
in  the  tegmentum  of  the  midbrain  ventral  to  the  inferior  colliculi,  and  end  in  the 
red  nucleus  and  thalamus  (Figs.  242,  243). 

Neuron  III. — From  cells  in  the  red  nucleus  arise  the  fibers  of  the  rubrospinal 
tract,  which  cross  the  median  plane  in  the  ventral  tegmental  decussation,  and 
descend  through  the  reticular  formation  of  the  brain  stem  and  the  lateral  funic- 
ulus of  the  spinal  cord.  Here  this  tract  occupies  a  position  just  ventral  to  the 
lateral  corticospinal  tract,  and  its  fibers  end  in  the  anterior  gray  column  in 
relation  to  the  primary  motor  neurons. 

We  have  learned  that  the  cerebellum  is  the  chief  center  of  the  proprioceptive 
system  and  is  concerned  with  the  maintenance  of  the  proper  tonicity  of  the 
muscles,  the  coordination  of  their  contractions,  and  especially  with  those  re- 
actions necessary  to  maintain  or  to  re-establish  that  evenly  balanced  spacial 
orientation  known  as  equilibrium.  The  cerebello-rubro-spinal  path  is  the  con- 
duction system  primarily  concerned  in  these  reactions. 

What  is  perhaps  the  first  direct  experimental  evidence  of  the  function  of  this 
system  has  been  given  by  Weed  (1914).     The  extensor  rigidity,  so  characteristic 


EFFERENT    I'A  HIS    AND    I- I  I  I  I  \    arcs 


327 


of  decerebrated  dogs,  which  Sherrington  (1906)  clearly  showed  to  be  a  proprio 
ceptive  reflex  that  under  normal  conditions  serves  to  keep  the  limbs  From  bend 
ing  under  the  weight  of  the  body,  is  apparently  dependent  upon  the  integrity  of 
the  cerebello-rubro-spinal  path.    Weed  showed  that  removal  of  the  cerebellum, 
section  of  the  superior  cerebellar  peduncles,  or  transection  of  the  mesencephalon 
below  the  level  of  the  red  nucleus  obliterated  or  greatly  decreased  this  rigidity. 


From  frontal  lobe  and  corpui  striatum 
Thalamus 


Rubrospinal  tract  ^ 
Rubroreticular  tract  -._ 


Red  nucleus 
Brachium  <  onjunciivum 

Dentate  nucleus 


Pons 

Rubrospinal  trad 

Medulla  oblongata 


Reticulospinal  tract 


Spinal  cord 


Fig.  243. — Diagram  showing  the  connections  of  the  red  nucleus:  A,  Ventral  tegmental 
decussation;  B,  decussation  of  the  brachium  conjunctivum;  C  and  D,  descending  fibers  from  bra- 
chium conjunctivum,  before  and  after  its  decussation  respectively. 

On  the  other  hand,  stimulation  of  the  area  occupied  by  the  red  nucleus  on  the 
cut  surface  of  the  mesencephalon  in  decerebrated  dogs  increased  the  rigidity. 


IMPORTANT  REFLEX  ARCS 

We  have  considered  the  afferent  paths  leading  to  the  cerebral  cortex  and  to 
the  cerebellum  as  well  as  the  efferent  channels  which  conduct  impulses  from  these 
centers  to  the  skeletal  musculature.  But  there  are  many  more  direct  paths 
by  which  impulses  may  travel  from  receptor  to  effector,  and  these  are  known  as 
reflex  arcs.  It  will  be  worth  while  to  review  briefly  a  few  of  the  more  important 
of  these  rather  direct  receptor  to  effector  circuits. 


328  CHE    NERVOUS    SYSTEM 

REFLEX  ARCS  OF  THE  SPINAL  CORD 

Neuron  I.  Primary  sensory  neurons,  with  cell  bodies  in  the  spinal  ganglia, 
convey  impulses  from  the  sensory  endings  to  the  spinal  cord,  then  along  the 
ascending  and  descending  branches  resulting  from  the  bifurcation  of  the  dorsal 
root  fibers  within  the  cord,  and  along  the  collaterals  of  these  branches  to  the 
primary  motor  neurons,  either  directly  or  through  an  intercalated  central  unit 
(Figs.  66-68). 

Neuron  II. — The  central  neurons  have  their  cell  bodies  in  the  posterior  gray 
column  and  may  belong  to  Golgrs  Type  II,  having  short  axons  restricted  to  the 
gray  matter;  or  their  axons  may  be  long,  running  through  the  fasciculi  proprii 
to  the  ventral  horn  cells  at  other  levels  of  the  cord.  Some  of  these  central  axons 
cross  the  median  plane  in  the  anterior  commissure. 

Neuron  III. — Primary  motor  neurons,  with  cell  bodies  in  the  anterior  gray 
column,  send  their  axons  through  the  ventral  roots  and  spinal  nerves  to  the 
skeletal  musculature.  Or  in  the  case  of  visceral  reflexes,  the  motor  neuron  has 
its  cell  body  located  in  the  intermediolateral  cell  column,  and  its  axon  runs  as  a 
preganglionic  fiber  to  a  sympathetic  ganglion,  whence  the  impulses  are  relayed 
by  a  fourth  or  postganglionic  neuron  to  involuntary  muscle  or  glandular  tissue. 

The  reflex  paths  of  the  cranial  nerves  are  similarly  constituted,  except  that 
rarely  if  ever  do  the  sensory  fibers  form  synapses  directly  with  the  motor  cells. 
The  central  neuron,  which  has  its  cell  located  in  the  sensory  nucleus  of  a  given 
nerve,  sends  its  axon  through  the  reticular  formation  to  the  motor  nucleus  of 
the  same  or  of  some  other  nerve  (Figs.  92,  111).  Two  of  the  reflex  circuits  con- 
nected with  the  vestibular  nerve  require  special  attention. 

VESTIBULAR  REFLEX  ARC  THROUGH  THE  MEDIAL  LONGITUDINAL  BUNDLE 

Neuron  I. — The  bipolar  cells  of  the  vestibular  ganglion  in  the  external  audi- 
tory meatus  send  peripheral  processes  to  the  cristas  of  the  semicircular  canals 
and  maculae  of  the  saccule  and  utricle.  Their  central  processes  run  through 
the  vestibular  nerve  to  the  vestibular  nuclei  (Figs.  135,  244). 

Neuron  II. — Cells  in  the  lateral  and  superior  vestibular  nuclei  send  their  axons 
to  the  medial  longitudinal  fasciculus  of  the  same  or  the  opposite  side,  where  they 
divide  into  ascending  and  descending  branches,  which  run  in  this  bundle.  From 
these  branches  twigs  are  given  off  to  the  nuclei  of  the  oculomotor,  trochlear,  and 
abducens  nerves  and  to  the  motor  cells  of  the  cervical  portion  of  the  spinal  cord 
(Fig-  244). 

Neuron  III. — Primary  motor  neurons  of  the  oculomotor,  trochlear,  abducens, 


EFFERENT  PATHS  AND  REFLEX  \r< 


329 


accessory,  and  cervical  spinal  nerves  send  their  axons  to  the  m  that  move 

tlic  head  and  eyes. 

This  arc  is  concerned  with  the  reflex  regulation  of  the  combined  movements 
of  the  head  and  eyes  in  response  to  the  vestibular  stimulation  u  hii  h  re  wit  -  from 
every  movement  and  change  of  posture  of  the  head.  Strong  stimulation  oi  the 
semicircular  canals,  vestibular  nerve,  or  Deiters'  nucleus  causes  an  oscillatory 
side  to  side  movement  of  the  eyes,  known  as  nystagmus,  a  reflex  response  of  an 
abnormal  character  mediated  through  this  arc  (Wilson  and  Pike,  1915). 

.1/.  rectus  medialis 
,      Oculomotor  nerve 


M .  rectus  lateralis 

Nut .  of  ot  ulomolor  nerve 
.  Ibducens  nerve 


Vestibular  nerve  ' 
Lateral  vestibular  ma  leus  - 

Vestibulospinal  tract " 

Median  longitudinal  - 
fasciculus 


M.  stemocleidomas- 
toideus 


-  .Xuc  of  abdui  t  ns  nerve 

Median  longitudinal 
fast  ii  al us 

Spinal  root  of  accessory 
nerve 


V.  cervicalis  II 


Fig.  244. — Vestibular  reflex  arcs.      (Modified  after  Edinger.) 

A  vestibulospinal  reflex  arc  is  established  between  the  vestibular  sense  organs 
and  the  skeletal  musculature  and  consists  of  the  following  parts:  the  vestibular 
nerve;  the  vestibulospinal  tract,  which  has  its  origin  in  the  lateral  vestibular 
nucleus,  and  descends  in  the  ventral  funiculus  of  the  same  side  of  the  spinal 
cord;  and  the  primary  motor  neurons  of  the  spinal  cord  (Fig.  244). 

The  afferent  impulses  reaching  the  medulla  oblongata  by  way  of  the  vagus 
give  rise  to  a  great  variety  of  reflexes.  While  these  are  for  the  most  part  purely 
visceral,  a  few  are  executed  by  the  somatic  musculature  and  should  receive 
attention  at  this  point. 


33° 


THE    NERVOUS    SYSTEM 


The  Respiratory  Reflex  Mechanism. — The  maintenance  of  the  normal  res- 
piratory rhythm  is  dependent  upon  a  respiratory  center  in  the  caudal  part  of 
the  medulla  oblongata,  which  is  sensitive  to  changes  in  the  carbon  dioxid  con- 
tent of  the  blood.  But  this  rhythm  is  also  influenced  by  afferent  impulses  coming 
from  the  lungs  by  way  of  the  vagus  nerve  and  the  tractus  solitarius.  It  is 
probable  that  these  impulses  are  relayed  through  the  nucleus  of  the  tractus  soli- 
tarius and  descending  fibers  that  arise  in  that  nucleus  (tractus  solitariospinalis) 
to  the  primary  motor  neurons  belonging  to  the  phrenic  and  intercostal  nerves 
(Fig.  245).  There  must  also  be  a  descending  tract  from  the  respiratory  center 
to  these  neurons.  Cajal  (1909)  believes  that  this  center  is,  in  fact,  identical 
with  the  lower  part  of  the  nucleus  of  the  tractus  solitarius  (the  commissural 


Dorsal  motor  X  nucleus 
Nucleus  offascic.  solitarius 

Fasciculus  solitarius 

Vagus  ganglion 

Vagus  nerve 

Tr.  solitariospinalis 

Sympathetic  ganglion  -i 


Lung 

Intercostal  nerve 

Intercostal  muscle 

Phrenic  nerve 


Blood-vessel 
Respiratory  center 


Diaphragm 


Fig.  245. — Reflex  mechanism  of  respiration.      (Herrick,  Cajal.) 

nucleus),  and  that  this  responds  both  to  changes  in  the  chemical  composition  of 
the  blood  and  to  the  afferent  impulses  coming  by  way  of  the  vagus  nerve.  If 
this  be  true,  the  fibers  from  the  nucleus  of  the  tractus  solitarius  would  be  the 
only  descending  tract  needed  to  carry  the  respiratory  impulses  to  the  spinal 
cord.  Although  on  its  afferent  side  the  respiratory  reflex  is  visceral,  it  is  ex- 
ecuted by  somatic  muscles  which  are  under  voluntary  control;  and  hence  breath- 
ing may  be  temporarily  suspended  or  the  rhythm  altered  at  will. 

The  reflex  mechanism  for  vomiting  and  coughing  is  illustrated  in  Fig.  246. 
As  the  result  of  an  irritation  of  the  gastric  mucous  membrane  a  wave  of  excitation 
travels  along  the  afferent  fibers  of  the  vagus  nerve  and  the  tractus  solitarius. 
After  passing  through  synapses  in  the  nucleus  of  that  tract,  the  impulses  probably 


I  I  II  1:1  \  I     PA  III>    AM)    K  1.1  1.1  \     mm  S 


travel  along  the  descending  fibers,  which  arise  in  that  nucleus,  to  the  primary 
motor  neurons  of  the  spinal  cord  that  give  rise  to  the  fibers  innervating  the  dia- 
phragm an<l  abdominal  muscles.  At  the  same  time  the  musculature  of  the 
stomach  is  excited  to  contraction  by  that  part  of  the  wave  of  excitation  which 
reaches  the  dorsal  motor  nucleus  of  the  vagus.  These  impulses  reach  the  mus- 
culature of  the  Stomach  over  the  visceral  efferent  fibers  of  the  vagus  and  an 
intercalated  postganglionic  neuron. 

A  similar  neural  circuit  is  probably  responsible  for  reflex  coughing.     From 
the  irritated  respiratory  mucous  membrane,  as,  for  example,  of  the  larynx,  the 


Vagus  ganglion 


Larynx 


Vagus  nerve 


Intercostal  muscle 


Diaphragm 


Stomach 


Dorsal  motor  vagus 
nucleus 

Nucleus  of  fasciculus 
solitarius 

Fasciculus   solitarius 

Tr.  solitariospinalis 


Phrcnit 

Intercostal  >: 

Nerve  to  abdominal 
muscles 

Sympathetic  ganglion 
Postganglionic 


Fig.  246. — Reflex  mechanism  of  coughing  and  vomiting.     (Herrick,  Cajal.) 

disturbance  is  propagated  along  the  afferent  fibers  of  the  vagus,  through  the 
nucleus  of  the  tractus  solitarius  and  the  descending  fibers  arising  in  it  to  the 
spinal  primary  motor  neurons,  which  innervate  the  diaphragm  and  the  inter- 
costal and  abdominal  muscles. 

The  corpora  quadrigemina  are  important  reflex  centers.  The  path  for  re- 
flexes in  response  to  sound  begins  in  the  spiral  organ  of  Corti  and  follows  the  coch- 
lear nerve  and  its  central  connections,  including  the  lateral  lemniscus,  to  the 
inferior  colliculus  of  the  opposite  side,  and  to  a  less  extent  of  the  same  side  also 


$3* 


THE    NERVOl  S    SYS'l  KM 


see  p.  309).  Thence  the  path  follows  the  tectospinal  and  tectobulbar  tracts 
to  the  primary  motor  neurons  of  the  cerebrospinal  nerves  (see  p.  167).  The 
visual  reflex  arc  begins  in  the  retina,  follows  the  optic  nerve  and  optic  tract  with 
partial  decussation  in  the  chiasma,  to  the  superior  colliculus  of  the  corpora 
quadrigemina  (p.  226) ;  thence  it  is  continued  by  way  of  the  tectospinal  and  tecto- 
bulbar paths  to  the  primary  motor  neurons  of  the  cerebrospinal  nerves  (Fig.  162). 
Pupillary  Reactions. — The  iris  is  innervated  by  two  sets  of  sympathetic 
nerve-libers  derived  from  the  ciliary  and  the  superior  cervical  sympathetic  ganglia 
respectively.  Impulses  reaching  the  iris  through  the  latter  ganglion  induce 
dilatation  of  the  pupil;  those  through  the  ciliary  ganglion  cause  constric+io~ . 
The  latter  reaction  always  accompanies  accommodation.     When  vision  is  fo- 


Sup.  colliculus 
Sensory  nuc.  .V.  V 

Pons  -- 


Upper  thoracic  segments  of  < 
spinal  cord 


N.  V 
\  Carotid  plexus 
Sup.  cervical  sympathetic  ganglion 
Cervical  sympathetic  trunk 


Fig.  247. — Pupillary  reflex  arcs. 


cused  on  a  near  object,  contraction  of  the  ciliary  muscle  results  in  accommoda- 
tion; and  at  the  same  time  contraction  of  the  two  internal  rectus  muscles  brings 
about  a  convergence  of  the  visual  axes.  These  two  movements  are  always 
associated  with  a  third,  the  contraction  of  the  sphincter  pupillae.  In  addition 
to  this  constriction  of  the  pupil,  which  accompanies  accommodation,  two  other 
pupillary  reactions  require  attention  (Fig.  247). 

The  Pupillary  Reflex  (Light  Reflex).— When,  light  impinges  on  the  retinae 
there  results  a  contraction  of  the  sphincter  pupillae  and  a  corresponding  constric- 
tion of  the  pupil.  The  reflex  circuit,  which  is  traversed  by  the  impulses  bringing 
about  this  reaction,  begins  in  the  retina  and  includes  the  following  elements: 
the  fibers  of  the  optic  nerve  and  tract,  with  a  partial  decussation  in  the  optic 


EFFERENT  PATHS  AND  REFLEX  ARCS 


333 


chiasma;  synapses  in  the  superior  colliculus  of  the  corpora  quadrigemina;  fibers 
of  the  tectobulbar  tract  ending  in  the  nucleus  of  Edinger-Westphal  (visceral 
efferent  portion  of  the  oculomotor  nucleus);  the  visceral  efferenl  fibers  of  the 
oculomotor  nerve,  ending  in  the  ciliary  ganglion;  and  the  postganglioni<  fibers 
extending  from  the  ciliary  ganglion  to  iris. 

The  pupillary-shin  reflex  is  a  dilatation  of  the  pupil  following  scratching  of 
the  skin  of  the  cheek  or  chin.  This  is  hut  one  example  of  the  fact  that  dilatation 
of  the  pupil  can  he  induced  by  the  stimulation  of  many  sensory  nerves  and  i  on 
stantly  occurs  in  severe  pain.  The  path  includes  the  following  parts:  the  fibers 
of  these  sensory  nerves  and  their  central  connections  in  the  brain  stem  and  spinal 
cord;  preganglionic  visceral  efferent  fibers,  which  arise  from  the  cells  of  the  inter- 
mediolateral  column  of  the  spinal  cord  and  run  through  the  upper  white  rami 
and  the  sympathetic  trunk  to  the  superior  cervical  sympathetic  ganglion;  and 
postganglionic  fibers,  which  arise  in  that  ganglion  and  run  through  the  plexus  on 
the  internal  carotid  artery  to  end  in  the  iris  (Fig.  247). 

We  have  in  the  case  of  the  pupillary  reactions  an  illustration  of  the  double 
and  antagonistic  innervation,  which,  as  we  shall  see  in  the  next  chapter,  is  a 
rather  characteristic  feature  of  the  autonomic  nervous  system. 


CHAPTER  XXI 

THE  SYMPATHETIC  NERVOUS  SYSTEM 

The  sympathetic  nervous  system  is  an  aggregation  of  ganglia,  nerves,  and 
plexuses,  through  which  the  viscera,  glands,  heart,  and  blood-vessels,  as  well  as 

Ciliary  ganglion       Maxillary  nerve 
Sphenopalatine  ganglion        v 
Superior  cervical  ganglion  of  sympathetic    \     \ 


Cervical  plexus 


Brachial  plexus 


Greater  splanchnic  nerve 
Lesser  splanchnic  nerve  - 


Lumbar  plexus 


Sacral  plexus 


■Pharyngeal  plexus 

Middle  cervical  ganglion  of  sympathetic 
Inferior  cervical  gang,  of  sympathetic 

Recurrent  nerve 
Bronchial  plexus 

Cardiac  plexus 

Esophageal  plexus 
Coronary  plexus 


Left  vagus  nerve 

Gastric  plexus 
Celiac  plex~us 

Superior  mesenteric  plexus 


—j- Aortic  plexus 

^s— Inferior  mesenteric  plexus 

Hypogastric  plexus 

Pelvic  plexus 

Bladder 
Vesical  plexus 


Fig.  248. — The  sympathetic  nervous  system.     (Schwalbe,  Herrick.) 

smooth  muscle  in  other  situations,  receive  their  innervation.     As  illustrated  in 
Fig.  248  it  is  widely  distributed  over  the  body,  especially  in  the  head  and  neck 


334 


Mil.    SYMPA  l  III   l  H      \l  l:\  OUS    SYS1  I.M  335 

and  in  the  thoracic  and  abdominal  cavities.  It  musl  tiol  be  too  sharply  de- 
limitated from  the  cerebrospinal  nervous  system,  sin<  e  it  contains  greal  numbers 
of  fibers  which  run  to  and  from  the  brain  and  spinal  cord.  For  example,  the 
vagus  nerve  containsmany  fibers  which  are  distributed  through  the  thoracic 
and  abominal  sympathetic  plexuses  for  the  innervation  of  the  viscera.  In  the 
same  way  the  spinal  nerves  are  connected  by  communicating  brani  hes  or  rami 
communicates  with  the  sympathetic  trunks. 

The  sympathetic  trunks  are  two  nerve  cords  which  extend  vertically  through 
the  neck,  thorax,  and  abdomen,  one  on  each  side  of  the  vertebral  column  Fig. 
248).  Each  trunk  is  composed  of  a  series  of  ganglia  arranged  in  linear  order 
and  bound  together  by  short  nerve  strands.  Every  spinal  nerve  is  connected 
with  the  sympathetic  trunk  of  its  own  side  by  one  or  more  gray  rami  commu- 
nicantes  through  which  it  receives  libers  from  the  sympathetic  trunk.  Fibers 
reach  this  trunk  from  the  thoracic  and  upper  lumbar  nerves  by  way  of  the  white 
rami  communicantes  (Fig.  257).  The  sympathetic  trunk  also  gives  off  branches 
which  enter  into  the  formation  of  the  nerve  plexuses  which  are  associated  with 
the  larger  arteries.  The  largest  of  these  plexuses  is  the  celiac,  which  is  associ- 
ated with  the  upper  portion  of  the  abdominal  aorta  and  its  branches.  In  this 
plexus  and  located  in  close  relation  to  the  abdominal  aorta  are  the  celiac, 
mesenteric,  and  aorticorenal  ganglia,  all  of  which  are  in  man  grouped  in  a  pair 
of  large  irregular  masses  designated  as  the  celiac  ganglia  and  placed  one  on 
either  side  of  the  celiac  artery  (Fig.  257).  The  sympathetic  ganglia  may  be 
grouped  into  three  series  as  follows:  (1)  the  ganglia  of  the  sympathetic  trunk, 
arranged  in  linear  order  along  each  side  of  the  vertebral  column  and  joined 
together  by  short  nerve  strands  to  form  the  two  sympathetic  trunks;  (2)  col- 
lateral ganglia,  arranged  about  the  aorta  and  including  the  celiac  and  mesenteric 
ganglia;  and  (3)  terminal  ganglia,  located  close  to  or  within  the  structures 
which  they  innervate.  As  examples  of  the  latter  group  there  may  be  men- 
tioned the  ciliary  and  cardiac  ganglia  and  the  small  groups  of  nerve-cells  in 
the  myenteric  and  submucous  plexuses  (Fig.  257). 

FUNDAMENTAL  FACTS  CONCERNING  VISCERAL  INNERVATION 

General  visceral  afferent  fibers  are  found  in  the  ninth  and  tenth  cranial 
nerves  and  in  many  of  the  spinal  nerves,  especially  in  those  associated  with  the 
white  rami  (thoracic  and  upper  lumbar  nerves)  and  in  the  second,  third,  and 
fourth  sacral  nerves.  These  afferent  fibers  take  origin  from  cells  in  the  cerebro- 
spinal ganglia  (Fig.  249).     From  these  ganglia  the  fibers  run  through  the  corres- 


336  THE    NERVOUS    SYSTEM 

ponding  cerebrospinal  nerves  to  the  sympathetic  nervous  system,  through  which 
they  pass  without  interruption  in  any  of  its  ganglia  to  end  in  the  viscera.  These 
fibers  are  of  all  >izes.  including  large  and  small  myelinated  fibers  and  many  which 
are  unmyelinated  (Chase  and  Ranson,  1914;  Ranson  and  Billingsley,  1918). 

The  afferent  impulses  mediated  by  these  fibers  serve  to  initiate  visceral  re- 
flexes, and  for  the  most  part  remain  at  a  subconscious  level.  Such  general  vis- 
ceral sensations  as  we  do  experience  are  vague  and  poorly  localized.  Tactile 
sensibility  is  entirely  lacking  in  the  viscera  and  thermal  sensibility  almost  so, 
although  sensations  of  heat  and  cold  may  be  experienced  when  very  warm  or 
cold  substances  enter  the  stomach  or  colon  (Carlson  and  Braafladt,  1915). 
Pain  cannot  be  produced  by  pinching  or  cutting  the  thoracic  or  abdominal 
viscera.  Acute  visceral  pain  may,  however,  be  caused  by  disease,  as  in  the  pas- 
sage of  a  stone  along  the  ureter. 

From  the  cerebrospinal  ganglia  the  visceral  afferent  impulses  are  carried  to  the  brain 
and  spinal  cord  by  the  sensory  nerve  roots.  The  relations  within  the  cerebrospinal  ganglia 
are  not  entirely  clear;  but  it  seems  probable  that  the  visceral  afferent  impulses  are  conducted 
through  the  ganglion  by  way  of  the  two  branches  of  the  typical  unipolar  sensory  neuron 
(Fig.  249).  Many  authors  believe  that  there  are  also  sensory  fibers  which  arise  from  cells 
in  the  sympathetic  ganglia  and  terminate  in  the  spinal  ganglia  in  the  form  of  pericellular 
plexuses  (Fig.  40,  C).  Through  these  plexuses  visceral  sensory  impulses  are  supposed  to  be 
transmitted  to  somatic  sensory  neurons  and  to  be  relayed  by  them  to  the  spinal  cord.  Since 
it  has  not  been  clearly  demonstrated  that  any  sensory  fibers  arise  from  cells  in  the  sym- 
pathetic ganglia,  this  interpretation  of  the  pericellular  plexuses  of  the  spinal  ganglia  must  be 
regarded  as  purely  hypothetic. 

Langley  (1903)  has  presented  strong  evidence  that  few  if  any  sensory  fibers  arise  in  the 
sympathetic  ganglia.  Physiologic  experiments  show  that  the  visceral  afferent  fibers  run  in 
the  white  rami,  yet  all  or  practically  all  of  the  fibers  of  a  white  ramus  degenerate  if  the  cor- 
responding spinal  nerve  is  severed  distal  to  the  spinal  ganglion.  Huber  (1913)  states  that 
"it  has  not  been  determined  that  the  fine  medullated  fibers  or  the  unmedullated  fibers  which 
appear  to  enter  the  spinal  ganglia  from  without  and  end  in  pericellular  plexuses  are.  in 
fact,  the  neuraxes  of  sympathetic  neurones."  The  hypothesis  that  these  pericellular  plexuses 
represent  the  termination  of  visceral  afferent  fibers  is,  therefore,  not  well  supported.  This 
subject  is  treated  in  more  detail  in  a  series  of  papers  on  the  sympathetic  nervous  system  by 
Ranson  and  Billingsley  (1918). 

Visceral  Efferent  Neurons.— The  general  visceral  efferent  fibers  of  the 
cerebrospinal  nerves  take  origin  from  cells  located  within  the  cerebrospinal  axis. 
They  do  not  run  without  interruption  to  the  structures  which  they  innervate; 
instead,  they  always  terminate  in  sympathetic  ganglia,  whence  the  impulses, 
which  they  carry,  are  relayed  to  their  destination  by  neurons  of  a  second  order 
(Fig.  249).  This  important  information  we  owe  to  Langley  (1900  and  1903), 
who  showed  that  the  injection  of  proper  doses  of  nicotin  into  rabbits  prevents 


Mil.    SYMPATH1  m     NERV01  -    SYSTEM 


the  passage  of  impulses  through  the  sympathetic  ganglia,  although  an  undi 
minished  reaction  may  be  obtained  by  stimulation  of  the  more  peripheral  -\m 
pathetic  nerves  By  a  long  series  of  experiments  Langle)  bas  shown  that  there 
are  always  two  and  probably  never  more  than  two  neurons  concerned  in  the 
conduction  of  an  impulse  from  the  central  nervous  system  to  smooth  muscle 
or  glandular  tissue.  The  neurons  of  the  first  order  in  this  series  arc  designated  as 
preganglionic,  those  of  the  second  order  as  postganglionic,  with  reference  to  tin- 
relation  which  they  bear  to  the  ganglion  containing  their  synap 

Preganglionic  neurons  have  their  cell  bodies  located  in  the  visceral  efferent 
column  of  the  cerebrospinal  axis.     The  cells  of  this  series  are  smaller  than  those 

Somatic  afferent  fiber       ,,       .      , 
.     Visceral  afferent  fiber  Dorsa^oot 

\ 

^ Spinal  ganglion 

Dorsal  ramus 


f  Ventral  ramus 


Ramus  cotntnunicans 


Sympathetic  ganglion 


-<.    Visceral  efferent  fiber  ,-     ,    ,       , 

■c        ,-      J-       .  A  \  cntral  root 

Somatic  efferent  fiber 

Postganglionic  fiber 


.  _  Viscus 


Fig.  249. — Diagrammatic  section  through  a  spinal  nerve  and  the  spinal  cord  in  the  thoracic  region 
to  illustrate  the  chief  functional  types  of  peripheral  nerve-fibers. 

of  the  somatic  motor  column  and  contain  less  massive  Nissl  granules.  From 
these  cells  arise  the  fine  myelinated  visceral  efferent  fibers  which  run  through 
the  cerebrospinal  nerves  to  the  sympathetic  nervous  system  and  terminate  in 
the  sympathetic  ganglia  (Fig.  249). 

Postganglionic  neurons  have  their  cell  bodies  located  in  the  sympathetic 
ganglia.  In  fact,  these  cells  with  their  dendritic  ramifications  and  the  terminal 
branches  of  the  preganglionic  fibers  synaptically  related  to  them  are  the  es- 
sential elements  in  the  sympathetic  ganglia.  Their  axons  for  the  most  part 
remain  unmyelinated  and  run  as  Remak  fibers  through  the  sympathetic  nerves 


33* 


THE    NERVOUS    SYSTEM 


o    > 
ir.  -= 

Si 

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I  III     SVMI'Allll    I  I.      NERVOUS    SYST]  \1 


and  plexuses,  to  end  in  relation  with  involuntary  muscle  or  glandular  ti 
A  very  few  postganglionic  fillers  acquire  delicate  myelin  sheaths. 

Three  streams  of  preganglionic  fibers  leave  the  cerebrospinal  axis  I 
The  cranial  stream  includes  the  general  visceral  efferenl  fibers  of  the  1 1  ulomotor, 
facial,  glossopharyngeal,  vagus,  and  accessory  nerves.  These  fibers  end  in  the 
terminal  ganglia,  already  mentioned,  which  are  located  close  to  or  within  the 
organ  which  they  innervate.  In  the  cervical  nerves  there  are  no  visceral  ef- 
ferent fibers,  the  cranial  stream  being  separated  from  the  next  by  a  rather  wide 
gap.  The  thoracicolumbar  stream  includes  the  fibers  which  arise  from  the  cells 
of  the  intermediolateral  column  of  the  spinal  cord  and  make  their  exit  through 
the  thoracic  and  first  four  lumbar  nerves  (Langley,  1892;  Miiller,  1000).  After 
leaving  the  spinal  nerves  by  way  of  the  white  rami  they  enter  the  sympathetic 
nervous  system  and  terminate  in  the  ganglia  of  the  sympathetic  trunk  or  in  the 
celiac  and  associated  collateral  ganglia  (Fig.  250).  The  sacfal  stream  includes 
the  visceral  efferent  fibers  of  the  second,  third,  and  fourth  sacral  nerves.  These 
arise  from  cells  in  the  lateral  column  of  gray  matter  in  the  sacral  portion  of  the 
spinal  cord  and  run  through  the  visceral  branch  of  the  third  sacral  and  a  similar 
branch  from  either  the  second  or  fourth  sacral  nerves.  These  fibers  end  in  the 
ganglia  of  the  pelvic  sympathetic  plexuses. 

The  Autonomic  Nervous  System. — For  many  reasons  it  is  convenient  to  have 
a  name  which  vill  designate  the  sum  total  of  all  general  visceral  efferent  neurons, 
both  preganglionic  and  postganglionic,  whether  associated  with  the  cerebral 
or  spinal  nerves.  For  this  purpose  the  term  "autonomic  nervous  system"  is 
in  general  use.  It  designates  that  functional  division  of  the  nervous  system 
which  supplies  the  glands,  heart,  and  smooth  musculature  with  their  efferent  in- 
nervation (Fig.  250).  It  is  important  to  bear  in  mind  that  this  is  a  functional 
and  not  an  anatomic  division  of  the  nervous  system,  that  it  includes  only  efferent^ 
elements,  and  that  the  preganglionic  neurons  lie  in  part  within  the  cerebrospinal 
nervous  system.  The  terminal  portions  of  the  preganglionic  fibers  and  the 
postganglionic  neurons  are  located  in  the  sympathetic  system.  According  to 
the  origin  of  the  preganglionic  fibers,  we  may  recognize  the  following  three 
subdivisions  of  the  autonomic  system:  (1)  the  cranial  autonomic  system,  whose 
preganglionic  fibers  make  their  exit  by  way  of  the  third,  seventh,  ninth,  tenth. 
and  eleventh  cranial  nerves;  (2)  the  thoracicolumbar  autonomic  system,  whose  pre- 
ganglionic fibers  make  their  exit  by  way  of  the  thoracic  and  upper  lumbar  spinal 
nerves;  and  (3)  the  sacral  autonomic  system,  whose  preganglionic  fibers  run  in 
the  visceral  rami  of  the  second,  third,  and  fourth  sacral  nerves  (Fig.  250). 


34Q 


THE    NERVOUS    SYSTEM 


The  fibers  of  the  thoracicoliimbar  stream  run  by  way  of  the  white  rami  to 
the  sympathetic  trunk,  while  the  libers  of  the  cranial  and  sacral  streams  make 
no  connection  with  that  trunk,  but  run  directly  to  the  sympathetic  plexuses. 
And  while  the  thoracicolumbar  preganglionic  fibers  terminate  in  the  ganglia  of 
the  trunk,  those  of  cranial  and  sacral  origin  end  in  the  terminal  ganglia.  In 
these  two  respects  the  cranial  and  sacral  streams  agree  with  each  other  and  differ 
from  the  thoracicolumbar  outflow.  Also  in  their  response  to  certain  drugs, 
like  atropin  and  adrenalin,  the  two  former  agree  with  each  other  and  differ  from 
the  latter.  It  is,  therefore,  desirable  to  group  the  cranial  and  sacral  systems 
together  as  the  craniosacral  autonomic  system.  This  has  been  called  by  many 
physiologists  the  parasympathetic  system.  It  stands  in  contrast  to  the  thoracico- 
lumbar autonomic  system  to  which  many  physiologists  have  unfortunately  applied 
the  name  "sympathetic  system."  The  importance  of  recognizing  these  two 
principal  subdivisions  is  further  emphasized  by  the  fact  that  most  of  the  struc- 
tures innervated  by  the  autonomic  system  receive  a  double  nerve  supply  and  are 
supplied  with  fibers  from  both  subdivisions.  The  thoracicolumbar  fibers  are 
accompanied  in  most  peripheral  plexuses  by  craniosacral  fibers  of  opposite  func- 
tion so  that  the  analysis  of  these  plexuses  is  greatly  facilitated  by  subdividing 
the  autonomic  system  in  this  way. 

Visceral  Reflexes. — In  the  gastro-intestinal  tract  and  perhaps  within  other 
viscera  there  may  be  a  mechanism  for  purely  local  reactions  as  indicated  in 
the  following  paragraph.  With  this  exception  the  evidence  strongly  indicates 
that  all  visceral  reflex  arcs  pass  through  the  cerebrospinal  axis.  In  such  an 
arc  there  are  at  least  three  neurons,  namely,  (1)  visceral  afferent,  (2)  pregang- 
lionic visceral  efferent,  and  (3)  postganglionic  visceral  efferent  neurons  (Fig.  249) . 

The  purely  local  reactions  which  occur  in  the  gut  wall  after  section  of  all  of 
the  nerves  leading  to  the  intestine  are  known  as  myenteric  reflexes  and  must  de- 
pend upon  a  mechanism  different  from  that  of  other  visceral  reflexes  (Langley 
and  Magnus,  1905;  Cannon,  1912).  Practically  nothing  is  known  of  this  mech- 
anism beyond  the  fact  that  it  must  be  located  in  the  enteric  plexuses.  Some 
authors  have  assumed  that  within  these  plexuses  there  is  a  diffuse  nerve  net 
similar  to  that  found  in  the  ccelenterates  (Parker,  1919).  While  the  evidence 
is  far  from  satisfactory,  it  may  be  that  such  a  net  does  exist  in  this  situation  and 
that  it  is  responsible  for  these  local  reactions. 


Mil:    SYMiw  illi;iK     NERVOUS    SYSTEM 
STRUCTURE  OF  THE  SYMPATHETIC  GANGLIA 


I  I),  nerve-cells  of  the  sympathetic  ganglia  arc  almosl  all  multipolar,  hut  there 

arc  also  a  feu  that  art-  unipolar  or  bipolar.      Each  <  ill  i    surrounded  l>\  a  leati  'I 

membranous  capsule.    Some  of  the  dendrites  ramify  beneath  tlii-  capsule  and 
are  designated  as  intracapsular.    Others  pierce  the  capsule,  run  Ion-  distances 

through  the  ganglia,  and  are  known  as  extracapsular  dendrites. 


Fig.  251. — Neurons  from  the  human  superior  cervical  sympathetic  ganglion  (pyridin-silver 
method):  A,  Three  nerve  cells  and  the  intercellular  plexus:  a,  unicellular  glomerulus;  b,  neuron 
with  extracapsular  dendrites.  B,  Tricellular  glomerulus.  C,  Neuron  surrounded  by  subcapsular 
dendrites. 

Intracapsular  dendrites  are  numerous  in  the  sympathetic  ganglia  of  man. 
but  rare  in  those  of  mammals  (Marinesco,  1906;  Cajal,  1911;  Michailow,  1911; 
Ranson  and  Billingsley,  1918).  Beneath  the  capsule  these  dendrites  may  form 
an  open  network  more  or  less  uniformly  distributed  around  the  cell  (Fig.  251 .  O. 
or  they  may  be  grouped  on  one  side  of  the  cell,  causing  a  localized  bulging  in 
the  capsule  (Fig.  251,  A,  a).  Such  a  localized  mass  of  subcapsular  dendrites 
with  interlacing  branches  is  known  as  a  glomerulus.  Following  Cajal's  classifi- 
cation we  may  distinguish  four  types  of  glomeruli  according  to  the  number  of 


342 


THE    NERVOUS    SYSTEM 


neurons  whose  dendrites  enter  into  their  formation,  namely,  unicellular  (Fig. 
251,  A ,  a),  bicellular,  tricellular  (Fig.  251,  B),  and  multicellular  glomeruli.  Short 
intracapsular  dendrites  with  swollen  ends  are  sometimes  present  in  the  sym- 
pathetic ganglia  of  mammals  (Fig.  252,  A). 


Fig.  252. — Sympathetic  ganglion  cells  showing  various  types  of  dendrites.  Redrawn  from 
Michailow.  Methylcne-blue  stain.  A,  From  superior  mesenteric  ganglion,  horse;  B,  from  celiac 
ganglion,  horse;  C,  from  stellate  ganglion,  horse;  D,  from  superior  cervical  ganglion,  dog;  E,  celiac 
ganglion,  horse;  F,  superior  cervical  ganglion,  dog. 

Extracapsular  dentrites  pierce  the  capsule,  run  for  longer  or  shorter  dis- 
tances among  the  cells,  and  help  to  form  an  intercellular  plexus  of  dendritic  and 
axonic  ramifications  (Fig.  251,  A).  These  dendrites  may  end  in  a  variety  of 
ways.  Some  of  these  types  of  endings  may  be  enumerated  as  follows:  (1) 
brush-like  endings  (Fig.  252,  A);   (2)  plate-like  or  bulbous  terminals   applied 


I  111     S\  Ml'\  I II I   in      M  l:\  I  ;  |\[ 


343 


against  the  outer  surf  ace  of  the  capsule  of  another  cell    I  '   /;  i        \   inter 

lacing  branches,  which  form  a  plexus  upon  the  outer  surface  of  the  capsule  of 
an  adjacent  cell  I  Fig.  252.  D 

Dogiel  (1896)  thought  that  the  cells  possessing  the  longest  dendriti  ry,  but 

Cajal  (,1'MI)  could  find  no  evidence  for  this,  and  was  unable  to  trace  anj  of  them  from  the 
ganglia  and  associated  nerves  to  the  viscera.  Carpenter  and  Cone!  (1914),  usinu  the  size 
and  arrangement  of  the  Nissl  granules  as  a  criterion,  were  able  t<>  find  only  one  cell  type  in 
the  sympathetic  ganglia,  and  concluded  that  these  ganglia  do  not  contain  sensory  nerve 


Fig.  253. — Neurons  and  intercellular  plexus  from  the  superior  cervical  sympathetic  ganglion  of  a 

dog  (pyridin-silver  method). 

The  axons  of  sympathetic  ganglion  cells  are  usually  unmyelinated,  but  a  few 
of  them  acquire  thin  myelin  sheaths.  They  are  the  postganglionic  libers  which 
relay  the  visceral  efferent  impulses  to  the  innervated  tissue.  According  to 
Cajal  (1911).  who  states  that  his  anatomic  studies  are  in  accord  with  the  physio- 
logic experiments  of  Langley,  the  axons  of  the  cells  in  the  ganglia  of  the  sympa- 
thetic trunk  dispose  themselves  in  one  of  the  three  following  ways:  1 1  Usually 
they  run  transversely  to  the  long  axis  of  the  ganglion  to  enter  a  gray  ramus. 


344 


THE    NERVOUS    SYSTEM 


(2)  The  axons  may  run  through  a  connecting  nerve  trunk  into  another  ganglion. 
He  is  not  able  to  say  whether  these  axons  only  run  through  the  second  ganglion 
or  whether  they  make  connections  with  its  cells.  In  the  chick  embryo  he  at  one 
time  described  collaterals  coming  from  those  longitudinal  fibers  of  the  ganglia, 
which  take  origin  in  neighboring  ganglia.  Now,  however,  he  is  inclined  to  doubt 
this  observation,  and  thinks  it  likely  that  these  collaterals  all  come  from  fibers 
that  have  entered  the  sympathetic  trunk  through  white  rami  at  other  levels. 


<?■£> 


Fig.   254. 


Fig.   255. 

Figs.  254  and  255. — Preganglionic  fibers  and  pericellular  plexuses  of  the  frog.  Fig.  254,  Pre- 
ganglionic fibers,  the  branches  of  which  form  pericellular  plexuses;  Fig.  255,  a  unipolar  sympathetic 
ganglion  cell  in  connection  with  which  a  preganglionic  fiber  is  terminating.  Methylene-blue. 
(Huber.) 

(3)  In  some  cases  the  axons,  arising  from  cells  in  the  ganglia  of  the  sympathetic 
trunk,  run  toward  the  neighboring  arteries  in  the  visceral  nerves. 

There  is  no  anatomic  evidence  worth  mentioning  in  favor  of  the  existence  of  association 
neurons,  uniting  one  sympathetic  ganglion  with  another  or  one  group  of  cells  with  another 
within  such  a  ganglion.  But  there  is  strong  physiologic  evidence  against  the  existence  of 
such  association  neurons  (Langley,  1900  and  1904);  and  Johnson  (1918)  has  shown  that  none 
are  present  in  the  sympathetic  trunk  of  the  frog. 


Termination  of  the  Preganglionic  Fibers. — The  spaces  among  the  cells  of  a 
sympathetic  ganglion  are  occupied  by  a  rich  intercellular  plexus  of  dendritic 


THE    SVMI'\    in     [C    NERVOT  s    SYSTEM 

branches  and  fine  axons  (Figs.  251,  A;  253).    The  fine  axons  represenl  therami 
fications  of  preganglionic  fibers  and   they  degenerate  when   the  connection 
between  the  ganglion   and  the  central    nervous   system  is  severed    (Ranson 
and   Billingsley,    L918).     Similar  fibers  pierce  the   capsules  surrounding  the 

cells  and  intertwine  with  the  intracapsular  dendrites.  No  doubt  synaptic 
relations  are  established  between  the  axonic  and  dendritic  ramifications  in 
these  plexuses. 

Another  and  very  characteristic  type  of  synapse  is  established  in  the  peri- 
cellular plexuses,  formed  by  the  terminal  ramifications  of  preganglionic  fibers  upon 
the  surface  of  the  cell  bodies  of  postganglionic  neurons.  Huber  (1899)  showed 
that  fibers  from  the  white  rami  branch  repeatedly  in  the  sympathetic  ganglia 
and  that  the  branches  terminate  in  subcapsular  pericellular  plexuses  (Figs.  254, 
255). 

In  the  sympathetic  ganglia  of  the  frog  the  pericellular  plexus  seems  to  be  the  only  type 
of  synapse  and  there  is  no  intercellular  plexus.  In  the  mammalian  sympathetic  ganglion 
these  pericellular  plexuses  are  harder  to  demonstrate  and  are  probably  less  numerous,  while 
the  intercellular  plexus  is  much  in  evidence.  It  is  well  established  that  one  preganglionic 
fiber  may  be  synaptically  related  to  several  postganglionic  neurons,  probably  in  some  in- 
stances to  as  many  as  thirty  or  more  (Ranson  and  Billingsley,  1918). 

COMPOSITION  OF  SYMPATHETIC  NERVES  AND  PLEXUSES 

Some  of  the  sympathetic  nerves  are  as  well  myelinated  as  the  cerebrospinal 
nerves  and  present  a  white  glistening  appearance.  This  is  true,  for  example,  of 
the  cervical  portion  of  the  sympathetic  trunk,  the  white  rami,  and  the  splanch- 
nic nerves.  Such  white  sympathetic  nerves  are  composed  at  least  in  large  part 
of  fibers  running  to  and  from  the  central  nervous  system.  Other  nerves  like 
the  gray  rami  and  branches  to  the  blood-vessels  are  gray,  because  they  are  com- 
posed chiefly  of  unmyelinated  postganglionic  fibers.  In  preceding  paragraphs 
we  have  shown  that  there  are  probably  no  association  or  sensory  neurons  in 
the  sympathetic  ganglia;  and,  if  this  be  true,  there  are  no  axons,  arising  from  such 
cells,  in  the  sympathetic  nerve  trunks  and  plexuses.  These  nerves  and  plexuses 
are  composed  of  the  following  three  kinds  of  fibers  (Fig.  256) :  (1)  Preganglionic 
visceral  efferent  fibers,  which  are  of  small  size  and  myelinated,  have  their  cells 
of  origin  in  the  cerebrospinal  axis,  and  terminate  in  the  sympathetic  ganglia. 
(2)  Postganglionic  fibers,  which  are  for  the  most  part  unmyelinated,  have  their 
cells  of  origin  in  the  sympathetic  ganglia  and  terminate  in  involuntary  muscle  or 
glandular  tissue.  (3)  Visceral  afferent  fibers,  which  include  myelinated  fibers 
of  all  sizes  as  well  as  many  that  are  unmyelinated,  have  their  cells  of  origin  in 


346 


THE   NERVOUS    SYSTEM 


the  cerebrospinal  ganglia  and  terminate  in  the  viscera.     The  statements  con- 
tained in  this  paragraph  should  not  be  applied  without  qualification  to  the  ter- 


S pi iinl  ganglion 
Dorsal  root 


Collateral  ganglion 


Gland-, 
Blood-vessel-, 


Pacinian  corpuscle 

n  smooth 
muscle 
Sensory  ending 


Motor  ending  on  smooth 
muscle'' 


Ventral  root 
Splanchnic  nerve 


//     ,.    Ganglion  of  sympathetic  trunk 
'•   fl^~--    Gray  ramus 
(JnM1^-'  "  White  ramus 


Sympathetic  trunk 

Dorsal  ramus 
■Ventral  ramus 
jg$  Gland 
~g?  Blood-vessel 

-  White  ramus 
v  Gray  ramus 
Ganglion  of  sympathetic  trunk 

Sympathetic  trunk 


Fig.  256. — Diagram  showing  the  composition  of  sympathetic  nerves.  Black  lines,  visceral 
afferent  fibers;  unbroken  red  lines,  preganglionic  visceral  efferent  fibers;  dotted  red  lines,  post- 
ganglionic visceral  efferent  fibers. 

minal  ganglia  and  plexuses,  since  it  is  probable  that  these  contain  additional 
elements  either  in  the  nature  of  sensory  neurons  or  of  a  nerve  net. 


ARCHITECTURE  OF  THE  SYMPATHETIC   NERVOUS  SYSTEM 

The  sympathetic  trunks  are  two  ganglionated  cords,  each  of  which  consists 
of  a  series  of  more  or  less  segmentally  arranged  ganglia,  bound  together  by  as- 
cending and  descending  nerve-fibers  and  extending  from  the  level  of  the  second 
cervical  vertebra  to  the  coccyx  (Figs.  248, 257) .  The  two  trunks  are  symmetrically 
placed  along  the  anterolateral  aspects  of  the  bodies  of  the  vertebrae.  There  are 
21  or  22  ganglia  in  each  chain;  and  of  these,  3  are  associated  with  the  cervical 
spinal  nerves,  10  or  11  with  the  thoracic,  4  with  the  lumbar,  and  4  with  the  sacral 
spinal  nerves.  The  sympathetic  trunks  are  connected  with  each  of  the  spinal 
nerves  by  one  or  more  delicate  nerve  strands,  called  rami  communicantes  (Figs. 


TILE   SYMPATHETIC    NERVOUS    SYSTEM 

248,  257).  T<>  each  spinal  nerve  there  runs  a  %ray  ramus  from  the  sympathetic 
trunk.  The  white  rami,  on  the  other  hand,  arc  more  limited  in  distribution  and 
unite  the  thoracic  and  upper  four  lumbar  nerves  with  the  corresponding  portion 
of  tlu-  sympathetic  trunk. 

The  white  rami  consist  of  visceral  afferent  and  preganglioni*  visceral  efferent 
fibers  directed  from  the  central  into  the  sympathetic  nervous  system.  The\ 
contribute  the  great  majority  of  the  ascending  and  descending  fibers  of  the 
sympathetic  trunk  (Fig.  257).  While  some  of  the  fibers  may  terminate  in  the 
ganglion  with  which  the  white  ramus  is  associated,  and  others  run  directlj 
through  the  trunk  into  the  splanchnic  nerves,  the  majority  of  the  fibers  turn 
either  upward  or  downward  in  the  trunk  and  run  for  considerable  distances  within 
it  (Fig.  250).  The  fibers  from  the  upper  white  rami  run  upward,  those  from  the 
lower  white  rami  downward,  while  those  from  the  intermediate  rami  may  run 
either  upward  or  downward.  The  cervical  portion  of  the  sympathetic  trunk 
consists  almost  or  quite  exclusively  of  ascending  fibers,  the  lumbar  and  sacral 
portions  of  the  trunk  largely  of  descending  fibers  from  the  white  rami.  The 
afferent  fibers  of  the  white  rami  merely  pass  through  the  trunk  and  its  branches 
to  the  viscera.  The  preganglionic  fibers,  with  the  exception  of  those  which  run 
out  through  the  splanchnic  nerves,  end  in  the  ganglia  of  the  trunk.  Here  they 
enter  into  synaptic  relations  with  the  postganglionic  neurons.  The  majority 
of  the  postganglionic  neurons,  located  in  the  ganglia  of  the  sympathetic  trunk, 
send  their  axons  into  the  gray  rami  (Figs.  250,  256). 

The  gray  rami  are  composed  of  postganglionic  fibers  directed  from  the  sym- 
pathetic trunk  into  the  spinal  nerves.  These  unmyelinated  fibers,  after  joining 
the  spinal  nerves,  are  distributed  with  them  as  vasomotor,  secretory,  and  pilo- 
motor fibers  to  the  blood-vessels,  the  sweat  glands,  and  the  smooth  muscle  of 
the  hair-follicles. 

Especially  in  the  cervical  region  there  are  other  important  branches  from  the 
sympathetic  trunk,  which  resemble  the  gray  rami  in  structure  and  which  convey 
postganglionic  fibers  to  certain  of  the  cranial  nerves  and  to  the  heart,  pharynx, 
the  internal  and  external  carotid  and  thyroid  arteries,  and  through  the  plexuses 
on  these  arteries  to  the  thyroid  gland,  salivary  glands,  eye,  and  other  structures 
(Figs.  248,  250,  257). 

The  cranial  portion  of  the  sympathetic  trunk  consists  of  three  ganglia  bound 
together  by  ascending  preganglionic  fibers  from  the  white  rami.  In  the  cat  it  has 
been  shown  to  contain  few  if  any  sensory  or  postganglionic  fibers.  The  superior 
cervical  ganglion  is  the  largest  of  the  three  ganglia  and  from  it  there  are  given  ofl 


348  mi:  nervous  system 

numerous  gray  nerve  strands.  These  are  all  composed  of  postganglionic  fibers 
which  arise  in  this  ganglion.  They  run  to  the  neighboring  cranial  and  spinal 
nerves,  to  which  they  carry  vasomotor,  pilomotor,  and  secretory  fibers,  and  to  the 
heart,  pharynx,  and  the  internal  and  external  carotid  arteries  (Figs.  248,  250. 
257).  The  most  important  of  these  branches  of  the  superior  cervical  ganglion 
are  the  three  following:  (1)  The  superior  cervical  cardiac  nerve,  which  run.-, 
from  the  superior  cervical  ganglion  to  the  cardiac  plexus,  carries  accelerator 
fibers  to  the  heart.  (2)  The  internal  carotid  nerve  runs  vertically  from  the 
ganglion  to  the  internal  carotid  artery,  about  which  its  fibers  form  a  plexu.-. 
known  as  the  internal  carotid  plexus  (Fig.  257).  It  is  by  way  of  this  nerve  and 
plexus  that  the  pupillary  dilator  fibers  reach  the  eye  (Fig.  247).  (3)  The  branch 
of  the  superior  cervical  ganglion  to  the  external  carotid  artery  breaks  up  into  a 
plexus  on  that  artery.  A  continuation  of  this  plexus  extends  along  the  external 
maxillary  artery,  and  carries  secretory  fibers  to  the  submaxillary  salivary  gland. 

The  middle  and  inferior  cervical  sympathetic  ganglia  are  smaller.  Among 
the  branches  from  these  ganglia  we  may  mention  the  gray  rami  to  the  adjacent 
spinal  nerves  and  the  middle  and  inferior  cardiac  nerves  to  the  cardiac  plexus 
(Figs.  248,  257; . 

The  thoracic  portion  of  the  sympathetic  trunk  i>  connected  with  the  thoracic 
nerves  by  the  gray  and  white  rami.  In  addition  to  the  rami  communicantes 
and  some  small  branches  to  the  aortic  and  pulmonary  plexuses,  there  are  three 
important  branches  of  the  thoracic  portion  of  the  sympathetic  trunk  known  as 
the  splanchnic  nerves.  These  run  through  the  diaphragm  for  the  innervation 
of  abdominal  viscera  (Figs.  248,  257).  The  greater  splanchnic  nerve  is  usually 
formed  by  branches  from  the  fifth  to  the  ninth  thoracic  sympathetic  ganglia 
and  after  piercing  the  diaphragm  joins  the  celiac  ganglion.  The  smaller  splanch- 
nic nerve  is  u.-ually  formed  by  branches  from  the  ninth  and  tenth  thoracic 
sympathetic  ganglia  and  terminates  in  the  celiac  plexus.  The  loiccrmost  splanch- 
nic nerve  arises  from  the  last  thoracic  sympathetic  ganglion  and  terminates  in 
the  renal  plexus.  These  splanchnic  nerves,  although  they  appear  to  be  branches 
of  the  thoracic  sympathetic  trunk,  are  at  least  in  major  part  composed  of  fibers 
from  the  white  rami,  which  merely  pass  through  the  trunk  on  their  way  to  the 
ganglia  of  the  celiac  plexus  (Figs.  250.  257:  Langley.  1900;  Ranson  and  Billings- 
ley.  1918  . 

THE  SYMPATHETIC  PLEXUSES 

The  Sympathetic  Plexuses  of  the  Thorax. — In  close  association  with  the 
vagus  nerve  in  the  thorax  are  three  important  sympathetic  plexuses.     The 


Mil      S"i  MI'MIII    I  [(       \l   l'\  .,1         3YST]  M  J4Q 

cardiac  plexus  lies  in  close  relation  to  the  arch  of  the  aorta,  and  from  it  sub- 
ordinate plexuses  arc  continued  along  the  coronary   arteries,     h   receives  the 
three  cardiac  sympathetic  nerves  from  the  cervical  portion  of  each  sympathetic 
trunk,  as  well  as  branches  from  both  vagus  nen  i      I      \.  248,  257).    Thepregan 
glionic  fibers  of  the  vagus  terminate  in  synaptic  relation  with  the  cells  of  the 
cardiac  ganglia.    They  convey  inhibitory  impulses  which  arc  relayed  through 
these  ganglia  to  the  cardiac  musculature  (Fig.  250).    The  cardia*   sympathetic 
nerves  contain  postganglionic  fibers  which  take  origin  in  the  cervical   sympa 
thetic  ganglia;  and  they  relay  accelerator  impulses,  coming  from  the  spinal  cord 
by  way  of  the  upper  white  rami  and  sympathetic  trunk  to  the  hearl  I  Fig.  250 
The  pulmonary  and  esophageal  plexuses  of  the  vagus  arc  also  to  be  regarded  as 
parts  of  the  sympathetic  system  (Fig.  257). 

The  celiac  plexus  (solar  plexus)  is  located  in  the  abdomen  in  close  relation 
to  the  celiac  artery  (Figs.  248,  257).  It  is  continuous  with  the  plexus  which 
surrounds  the  aorta.  Subordinate  portions  of  the  celiac  plexus  accompany 
the  branches  of  the  celiac  artery  and  the  branches  from  the  upper  part  of  the 
abdominal  aorta.  These  are  designated  as  the  phrenic,  suprarenal,  renal, 
spermatic  or  ovarian,  abdominal  aortic,  superior  gastric,  inferior  gastric,  he- 
patic, splenic,  superior  mesenteric,  and  inferior  mesenteric  plexuses.  The  celiac 
plexus  contains  a  number  of  ganglia  which  in  man  are  grouped  into  two  large 
flat  masses,  placed  one  on  either  side  of  the  celiac  artery  and  known  as  the 
celiac  ganglia.  These  ganglia  are  bound  together  by  strands  which  cross  the 
median  plane  above  and  below  this  artery.  Somewhat  detached  portions  of 
the  celiac  ganglion,  which  lie  near  the  origin  of  the  renal  and  superior  mesenteric 
arteries,  are  known  respectively  as  the  aorticorenal  and  superior  mesenteric 
ganglia.  In  addition,  there  is  a  small  mass  of  nerve-cells  in  the  inferior  mesen- 
teric plexus  close  to  the  beginning  of  the  inferior  mesenteric  artery.  This  is 
known  as  the  inferior  mesenteric  ganglion. 

Preganglionic  fibers  reach  the  celiac  plexus  from  two  sources,  namely,  from 
the  white  rami  by  way  of  the  sympathetic  trunk  and  splanchnic  nerves  and  from 
the  vagus  nerve  (Fig.  257).  Most  if  not  all  of  the  preganglionic  fiber-  contained 
in  the  splanchnic  nerves  terminate  in  the  ganglia  of  the  celiac  plexus.  At  the 
lower  end  of  the  esophageal  plexus  the  fibers  from  the  right  vagus  nerve  become 
assembled  into  a  trunk  which  passes  to  the  posterior  surface  of  the  stomach  and 
the  celiac  plexus.  The  fibers  of  the  left  vagus  pass  to  the  anterior  surface  of 
the  stomach  and  to  the  hepatic  plexus  (Fig.  257).  It  i>  probable  that  the  pre- 
ganglionic fibers  of  the  vagus  do  not  terminate  in  the  ganglia  of  the  celiac  plexus, 


6^ 


THE    NERVOUS    SYSTEM 


N.  VII 

I 
i 


Internal  carol  id  plexus. 
To  N.  X 
ToN.IX- 
To  cervu  al  X .  I  ■ 
X.  II 
X.  Ill 
X.IY—  -^* 

N.  V ^4 

N.  VI-  --*-»  ^ 

N.  VII. 

N.  VIII- 

To  thoracic  N.  I  • 

N.  II- 

N.  III- 

N.  IV- 

N.  V- 

N.  VI- 

N.  VII- 

N.  VIII- 

N.  IX- 

N.  X- 

N.  XI. 

N.  XII- 

To  lumbar  X.  /_ 
N.  II. 

N.  III- 
X.  IV- 
N.  V--- 

To  sacral  X.  I \^ 

N.  II , 

N.  Ill 
Visceral  branches  of  \rrT 
sacral  nerves  j111'  I  v  ^- . 

N.  IV  — 

N.  V  — 
To  coccygeal  nerve  <•**' 


-.V.  /// 

"Ciliary  ganglion 
~  S  plena  palatine  ganglion 
-N.  IX 
■•Otic  ganglion 
'Superior  cervical  ganglion 
"Pharyngeal  plexus 
-N.  VII 

-Submaxillary  ganglion 
'Middle  cervical  ganglion 
-Superior  cardiac  N. 
-Middle  cardiac  N. 
•Inferior  cardiac  N. 
■  Cardiac  brandies  of  vagus 
'  Vagus  and  left  pulmonary  plexus 
-Cardiac  plexus 
•Left  coronary  plexus 
.  Esophageal  plexus 
Splanchnic  nerves 

^Hepatic  plexus 
Left  vagus  nerve 


^    .Gastric  plexus 

\ Myenteric  and  sub- 

/^       mucous  plexuses 

'^^~  Splenic  plexus 
Celiac  plexus 
Superior  mesenteric 
plexus 

Inferior  mesenteric  plexus 


"Abdominal  aortic  plexus 
Hypogastric  plexus 


Fig.  257.— Diagram  of  the  sympathetic  nervous  system.  The  red  lines  indicate  the  branches 
of  the  cerebrospinal  nerves  which  join  the  sympathetic  system  and  those  sympathetic  nerves  which 
are  composed  in  major  part  of  fibers  from  the  cerebrospinal  nerves.  (Modified  from  Jackson- 
Morns.) 

but  merely  pass  through  that  plexus  to  end  in  the  terminal  ganglia,  such  as  the 
small  groups  of  nerve-cells  in  the  myenteric  and  submucous  plexuses  of  the  in- 
testine (Fig.  250). 


IHE    SYMPATH]   I  [I     NERV01  S    SYS1  l.M 


The  myenteric  plexus  (of  Auerbach)  and  the  submucous  plexus  (of  Mei  sner), 
located  within  the  walls  of  the  stomach  and  intestini  .«•  filaments  from 

the  gastric  and  mesenteric  divisions  of  the  celiac  plexus.  The]  also  receive 
fibers  from  the  vagus  either  directly,  as  in  the  case  of  the  stoma*  h.  or  indire*  tly 
through  the  celiac  plexus  (Fig.  257).  Unfortunately,  very  little  is  known  con- 
cerning the  synaptic  relations  established  in  the  ganglia  of  these  plexuses.  A. 
cording  to  Langley,  the  postganglionic  fibers  from  the  celiac  ganglia  run  through 
these  plexuses  without  interruption  and  end  in  the  muscular  coats  and  glands 
of  the  gastro-intestinal  tract.  The  preganglionic  fibers  from  the  vagus  probably 
end  in  synaptic  relation  to  cells  in  these  small  ganglia;  and  the  axons  of  these 
cells  serve  as  postganglionic  fibers,  relaying  the  impulses  from  the  vagus  to  the 
glands  and  muscular  tissue.  As  was  indicated  in  a  preceding  paragraph,  the 
enteric  plexuses  must  also  contain  a  mechanism  for  purely  local  reactions,  sun  e 
peristalsis  can  be  set  up  by  distention  in  an  excised  portion  of  the  gut.  But 
as  yet  we  are  entirely  ignorant  as  to  what  that  mechanism  may  be. 

The  hypogastric  plexus  is  formed  by  strands  which  run  into  the  pelvis  from 
the  lower  end  of  the  aortic  plexus  and  are  joined  by  the  visceral  branches  of  the 
second,  third,  and  fourth  sacral  nerves  and  by  branches  from  the  sympathetic 
trunk  (Figs.  248,  257).  As  the  hypogastric  plexus  enters  the  pelvis  it  splits  into 
two  parts,  which  lie  on  either  side  of  the  rectum  and  are  sometimes  called  the 
pelvic  plexuses.  From  these  plexuses  branches  are  supplied  to  the  pelvic  vis- 
cera and  the  external  genitalia. 

The  Cephalic  Ganglionated  Plexus. — In  close  topographic  relation  to  the 
branches  of  the  fifth  cranial  nerve  are  four  sympathetic  ganglia,  known  as  the 
ciliary,  sphenopalatine,  otic,  and  submaxillary  ganglia.  Each  of  these  is  con- 
nected with  the  superior  cervical  sympathetic  ganglion  by  filaments  derived 
from  the  plexuses  on  the  internal  and  external  carotid  arteries  and  their  branches 
(Fig.  257).  These  filaments  are  designated  in  descriptive  anatomy  as  the  sym- 
pathetic roots  of  the  ganglia.  Each  ganglion  receives  preganglionic  fibers  from 
one  of  the  cranial  nerves  by  way  of  what  is  usually  designated  as  its  motor  root 
(Fig.  257).  Thus  the  ciliary  ganglion  receives  fibers  from  the  oculomotor  nerve; 
the  sphenopalatine  ganglion  receives  fibers  from  the  facial  nerve  by  way  of  the 
great  superficial  petrosal  nerve  and  the  nerve  of  the  pterygoid  canal;  the  otic 
ganglion  receives  fibers  from  the  glossopharyngeal  nerve  (Miiller  and  Dahl,  1910) ; 
and  the  submaxillary  ganglion  receives  fibers  from  the  facial  nerve  by  way  of 
the  nervus  intermedius  and  the  Ungual  nerve.  Postganglionic  fibers  arising 
in  these  ganglia  are  distributed  to  the  structures  of  the  head.     From  the  ciliary 


352  THE   NERVOUS   SYSTEM 

ganglion  libers  go  to  the  intrinsic  musculature  of  the  eye.  Some  of  the  fibers 
arising  in  the  sphenopalatine  ganglion  go  to  the  blood-vessels  in  the  mucous 
membrane  of  the  nose.  Fibers  from  the  otic  ganglion  reach  the  parotid  gland. 
And  those  arising  in  the  submaxillary  ganglion  end  in  the  submaxillary  and 
sublingual  salivary  glands  (Fig.  250). 

IMPORTANT   CONDUCTION    PATHS    BELONGING   TO   THE  AUTONOMIC   NERVOUS 

SYSTEM 

Thanks  to  the  work  of  Langley,  we  know  that  the  conduction  pathways  in 
the  sympathetic  nervous  system  are  at  least  as  sharply  defined  as  those  in  the 
brain  and  spinal  cord.  A  great  deal  has  already  been  done  in  the  way  of  tracing 
these  pathways;  and  some  of  the  more  important  of  these  are  given  in  the  out- 
line which  follows : 

1.  Paths  for  the  efferent  innervation  of  the  eye  (Figs.  247,  250): 

(a)  Ocular  craniosacral  pathway. 

Preganglionic  neurons:  Cells  in  the  Edinger-Westphal  nucleus, 
fibers  by  way  of  the  third  cranial  nerve  to  end  in  the  ciliary  ganglion. 

Postganglionic  neurons:  Cells  in  the  ciliary  ganglion,  fibers  by 
way  of  the  short  ciliary  nerves  to  the  ciliary  muscle  and  the  circular 
fibers  of  the  iris. 

Function:    Accommodation  and  contraction  of  the  pupil. 

(b)  Ocular  thoracicolumbar  pathway. 

Preganglionic  neurons:  Cells  in  the  intermediolateral  column  of 
the  spinal  cord,  fibers  by  way  of  the  upper  white  rami  and  sympathetic 
trunk  to  end  in  the  superior  cervical  ganglion. 

Postganglionic  neurons:  Cells  in  the  superior  cervical  ganglion, 
fibers  by  way  of  the  internal  carotid  plexus  to  the  ophthalmic  division 
of  the  fifth  nerve,  the  nasociliary  and  long  ciliary  nerves  of  the  eyeball; 
other  fibers  pass  from  the  internal  carotid  plexus  through  the  ciliary 
ganglion,  without  interruption,  into  the  short  ciliary  nerves  and  to 
the  eyeball. 

Function:  Dilatation  of  the  pupil  by  the  radial  muscle-fibers  of 
the  iris. 

2.  Paths  for  the  efferent  innervation  of  the  submaxillary  gland  (Fig.  250): 
(a)  Submaxillary  craniosacral  pathway. 

Preganglionic  neurons:  Cells  in  the  nucleus  salivatorius  superior, 
fibers  by  way  of  the  seventh  cranial  nerve,  chorda  tympani,  and 


mi     SYMPATH]   i  K     NERV01  5    SYST]  M 

lingual  nerve  to  end   in  the  portion  of  the  submaxillary  ganglion 
located  on  the  submaxillary  duel . 

Postganglionic  neurons:  Cells  in  a  number  of  groups  along  the 
chorda  tympani  fibers  as  they  follow  the  submaxillary  duct,  fibers 
distributed  in  branches  to  the  submaxillar)  gland. 

Function :    [ncreases  se<  retion. 
(b)  Submaxillary  thoracicolumbar  pathway. 

Preganglionic  neurons:  Cells  in  the  intermediolateral  column  of 
the  spinal  cord,  fibers  by  way  of  the  upper  white  rami,  and  the  sym- 
pathetic trunk  to  end  in  the  superior  cervical  ganglion. 

Postganglionic  neurons:  Cells  in  the  superior  cervical  ganglion, 
fibers  by  way  of  the  plexuses  on  the  external  carotid  and  external 
maxillary  arteries  to  the  submaxillary  gland. 

Function:    Increases  secretion. 

3.  Paths  for  the  efferent  innervation  of  the  heart: 

(a)  Cardiac  craniosacral  pathway. 

Preganglionic  neurons:  Cells  in  the  dorsal  motor  nucleus  of  the 
vagus,  fibers  through  the  vagus  nerve  to  the  intrinsic  ganglia  of  the 
heart,  in  which  they  end. 

Postganglionic  neurons:  Cells  in  the  intrinsic  cardiac  ganglia, 
fibers  to  the  cardiac  muscle. 

Function:    Cardiac  inhibition. 

(b)  Cardiac  thoracicolumbar  pathway. 

Preganglionic  neurons:  Cells  in  the  intermediolateral  column  of 
the  spinal  cord,  fibers  by  way  of  the  upper  white  rami  and  the  sym- 
pathetic trunk  to  the  superior,  middle,  and  inferior  cervical  ganglia. 

Postganglionic  neurons:  Cells  in  the  cervical  ganglia  of  the  sym- 
pathetic trunk,  fibers  by  way  of  the  corresponding  cardiac  nerves  to 
the  musculature  of  the  heart. 

Function:    Cardiac  acceleration. 

4.  Paths  for  the  efferent  innervation  of  the  musculature  of  the  stomach 

exclusive  of  the  sphincters  (Fig.  250) : 
(a)  Gastric  craniosacral  pathway. 

Preganglionic  neurons:  Cells  in  the  dorsal  motor  nucleus  of  the 
vagus,  fibers  by  way  of  the  vagus  nerve,  to  end  in  the  intrinsic  ganglia 
of  the  stomach. 


23 


354 


THE    NERVOUS    SYSTEM 


Postganglionic  neurons:    Cells  in  the  intrinsic  gastric  ganglia,  fibers 
to  end  in  the  gastric  musculature. 
Function:    Excites  peristalsis. 
(b)  Gastric  thoracicolumbar  pathway. 

Preganglionic  neurons:  Cells  in  the  intermediolateral  column  of  the 
spinal  cord,  fibers  by  way  of  the  white  rami  from  the  fifth  or  sixth  to 
the  twelfth  thoracic  nerves,  through  the  sympathetic  trunk  without 
interruption,  and  along  the  splanchnic  nerves  to  the  celiac  ganglion, 
where  they  end. 

Postganglionic  neurons:  Cells  in  the  celiac  ganglion,  libers  by  way 
of  the  celiac  plexus  and  its  offshoots  to  the  stomach,  to  end  in  the 
musculature  of  the  stomach. 

Function:    Inhibits  peristalsis. 
5.  Paths  for  the  efferent  innervation  of  the  musculature  of   the  urinary 
bladder. 

(a)  Vesical  craniosacral  pathway. 

Preganglionic  neurons:  Cells  in  the  lateral  part  of  the  anterior 
gray  column  in  the  sacral  portion  of  the  spinal  cord,  fibers  by  way 
of  the  second  and  third  sacral  nerves  and  their  visceral  rami  through 
the  pelvic  plexus  to  the  plexus  upon  the  wall  of  the  bladder. 

Postganglionic  neurons:  Cells  in  the  small  ganglia  of  the  vesical 
plexus,  fibers  to  the  vesical  musculature. 

Function:  Excites  contraction  of  the  vesical  musculature  exclusive 
of  the  internal  sphincter  (trigonal  area),  the  contraction  of  which  it 
inhibits  and  thus  produces  urination. 

(b)  Vesical  thoracicolumbar  pathway. 

Preganglionic  neurons:  Cells  in  the  caudal  part  of  the  intermedio- 
lateral cell  column,  fibers  by  way  of  the  lower  white  rami  to  the  infe- 
rior mesenteric  ganglion. 

Postganglionic  neurons:  Cells  in  the  inferior  mesenteric  ganglion, 
fibers  through  the  inferior  mesenteric  plexus  to  the  musculature  of 
the  bladder. 

Function:     Excites  contraction  of  the  internal  sphincter  (trigonal 
area  of  the  vesical  musculature),  causing  retention  of  urine. 
It  will  be  noted  that  the  viscera  receive  a  double  autonomic  innervation,  and 
that  the  impulses  transmitted  along  the  craniosacral  pathways  are  usually 
antagonistic  to  those  transmitted  along  the  thoracicolumbar  paths. 


A  LABORATORY  OUTLINE  OF  NEURO-ANATOMY 

The  following  directions  for  the  study  of  the  gross  and  microscopi<   anatomy  of 

the  nervous  system  are  intended  to  aid  the  student  in  making  the  besl  use  of  bis  time 
and  laboratory  material.  Free  use  is  made  of  the  sheep's  brain  because  in  mosl  in- 
stitutions the  number  of  human  brains  available  is  limited,  and  these  arc  often  poorly 
preserved  and  entirely  unsuited  for  dissection.  Even  if  an  unlimited  supply  of  well- 
preserved  human  brains  were  at  hand,  there  would  still  be  an  advantage  in  the  use  of 
the  sheep's  brain  because  in  it  certain  structures  (such  as  the  olfactory  trai  1  -  and  <  enters 
and  the  really  significant  subdivisions  of  the  cerebellum)  are  more  easily  seen  and  more 
readily  understood. 

The  outline  has  been  written  in  such  a  way  that  it  can  be  readily  adapted  by  the 
instructor  to  meet  his  own  needs.  It  is  assumed  that  each  instructor  will  furnish  his 
students  with  a  schedule  for  the  laboratory  work,  showing  the  number  of  laboratory 
periods  available  and  the  topics  to  be  covered  each  period.  This  will  help  the  student 
properly  to  apportion  his  time  and  enable  the  instructor  to  arrange  the  order  of  the 
laboratory  work  to  his  own  liking.  The  paragraphs  have  been  numbered  serially  in 
order  that  in  such  a  schedule  they  may  be  referred  to  by  number.  It  is  not  necessary 
that  the  topics  be  taken  up  in  their  numeric  order.  And  in  a  course  of  one  hundred 
hours  some  of  the  topics  should  be  omitted  altogether.  How  much  should  be  omitted 
will  depend  largely  on  the  amount  of  drawing  required.  It  is  assumed  that  the  in- 
structor will  indicate  on  the  laboratory  schedule  the  drawings  which  he  wishes  to  have 
made.  For  this  reason  we  have,  for  the  most  part,  omitted  specific  directions  for  draw- 
ings. 

Since  it  will  be  necessary  for  the  student  in  using  the  outline  to  make  frequent 
references  to  figures  in  the  text,  it  will  be  convenient  to  keep  in  the  book  several  strips 
of  thin  paper  to  serve  as  bookmarks. 

METHODS  OF  BRAIN  DISSECTION 

Much  information  concerning  the  gray  masses  and  fiber  tracts  of  the  brain  can  be 
obtained  by  dissection.  This  should  be  carried  out,  for  the  most  part,  with  blunt 
instruments.  It  is  rarely  necessary  to  make  a  cut  with  a  knife.  An  orangewood  mani- 
cure stick  makes  an  excellent  instrument.  It  should  be  rounded  to  a  point  at  one  end 
for  teasing,  while  the  larger  end  should  be  adapted  for  scraping  away  nuclear  masses. 
A  pair  of  blunt  tissue  forceps  of  medium  size  with  smooth  even  edges  and  fine  transverse 
interlocking  ridges  is  also  an  essential  instrument.  This  i<  useful  in  grasping  and  -trip- 
ping away  small  bundles  of  fibers.  In  dissecting  out  a  fiber  tract  it  is  necessary  to  have 
in  mind  a  clear  idea  of  the  position  and  course  of  the  tract,  and  the  dissecting  instru- 
ments should  be  carried  in  the  direction  of  the  fibers.  Where  it  is  uecessary  to  remove 
nuclear  material  in  order  to  display  fiber  bundles,  it  will  be  found  very  helpful  to  let  a 
stream  of  water  run  over  the  specimen  while  the  dissection  is  in  progn 

355 


356  THE    NERV01  -    SYSTEM 

DISSECTION  OF  THE  HEAD  OF  THE  DOGFISH 

1.  The  dogfish  is  the  smallest  of  the  sharks.  Either  the  spiny  rlr^fish  (Squalus 
acanthias    or  the  smooth  dogfish  (Mustelus  canis)  may  be  used  for  dissection. 

2.  The  special  ■  <n-r  organs  include  the  olfactory  organs,  the  eyes,  the  ears,  and 
certain  sense  organs  in  the  skin,  known  as  the  lateral  line  canals,  and  the  ampulla-  of 
Lorenzini. 

\.  [xx  ate  the  position  of  the  lateral  line  canal  which  produces  a  light  colored  ridge 
in  the  skin  extending  from  head  to  tail  along  either  side  of  the  body.  The  line  may  be 
recognized  by  the  presence  of  numerous  small  pores  which  open  into  the  canal.  It 
extends  on  to  the  head  and  there  forms  the  supraorbital,  infraorbital,  and  hyoman- 
dibular  canals.  The  ampulla  of  Lorenzini  are  bulb-shaped  bodies  connected  by  long 
canals  with  pores  in  the  skin.  They  are  irregularly  arranged  and  are  most  numerous 
on  the  snout. 

4.  Locate  the  olfactory  organs  or  nasal  capsules  which  have  their  openings  on  the 
ventral  surface  of  the  snout  in  front  of  the  mouth. 

5.  Xote  the  gills  and  spiracles  (Fig.  12).  Find  two  minute  apertures  near  the 
midline  between  the  spiracles.     These  are  the  openings  of  the  ewlolymphatic  ducts. 

6.  The  internal  ear,  a  membranous  labyrinth  inclosed  in  a  cartilaginous  capsule, 
should  be  exposed  on  the  left  side.  Shave  <>\\  the  cartilage  in  thin  slices  in  the  region 
between  the  spiracle  and  the  median  plane.  The  membranous  labyrinth  can  be  seen 
through  the  translucent  cartilage,  and  care  should  be  exercised  to  avoid  injuring  it  while 
the  cartilage  is  being  removed.  It  consists  of  a  spheric  sac,  the  utriculosaccular  chamber, 
to  which  there  are  attached  three  semicircular  canals  (Fig.  12).  The  endolymphatic 
duct  is  a  small  canal,  which  extends  from  this  chamber  through  the  roof  of  the  skull  to 
the  small  opening  in  the  skin,  which  has  previously  been  identified.  Xote  the  enlarge- 
ment at  one  end  of  each  semicircular  canal,  known  as  the  ampulla,  and  observe  that 
each  of  these  canals  lies  in  a  plane  at  right  angles  to  the  planes  of  the  other  two. 

7.  The  Brain  and  Cranial  Serves. — Remove  the  remainder  of  the  roof  of  the  skull 
and  expose  the  brain,  eyes,  and  cranial  nerves. 

8.  Examine  the  brain  as  seen  from  the  dorsal  surface.  Note  the  continuity  of  the 
medulla  oblongata  with  the  Spinal  cord.  Identify  the  cerebellum,  the  thalamus,  epiphysis, 
habenula,  cerebral  hemispheres,  and  olfactory  bulbs  (Fig.  8  and  pp.  26-31  J. 

9.  By  dissection  display  on  the  left  side  the  eye-muscles  and  the  nerves  which  in- 
nervate them,  as  well  as  the  optic  nerve  (Fig.  12). 

10.  Find  the  nervus  tmninalis  (Fig.  8).  Now  locate  each  of  the  cranial  nerves 
from  the  second  to  the  tenth  inclusive,  and  trace  them  from  the  brain  as  far  as  possible 
toward  their  peripheral  terminations  (Figs.  12,  13).  Note  particularly  that  Nn.  VII 
and  X  each  have  an  extra  root,  indicated  in  black  in  Pig  13,  which  carries  fibers  from 
the  lateral  line  organs  to  the  acusticolateral  area  of  the  medulla. 

11.  Attention  should  now  be  paid  to  the  functional  types  of  nerve-fibers  which 
compo>e  each  of  the  cranial  nerves  (see  pp.  168  170  and  Figs.  119,  120).  The  ac- 
companying table  shows  in  which  of  the  cranial  nerves  of  the  dogfish  each  of  the  four 
principal  functional  groups  of  liber-  are  to  be  found  (Herrick  and  Crosby,  1918). 


\    LABORATORY    01   I  LIN]     l  IF    \i  I  RO   \\  \  \<>\\\ 

(   u  \\i  \i     \i  u\  i     (  OMPONl  NTS  Ol     mi     I i  i^n 


- 


Somal ic  sensory. 

Somatit  motoi 

Viscei 

■l..r. 

II.  Optic 

1.  <  llfai  torj 

III.    Muscle  sense 

1\  .  Muscle  sense 

III.  To  eye-muscles 

I V.  To  eye-mua  les 

III.  For  inl r i 1 1 -i»  mi 
ol  thi 

Y.  ( ieneral  cutaneous 
VI.  Muscle  sense 

VI.  To  eye-muscles 

V.  To  i  he  jaw  must  les 

\'l  1.  Lateral  line  fibers 

VII.  ( ieneral  \  is*  eral 

VII.  To  hyoid   must  ula- 

VIII.  To  the  ear 

.tnd  gustatory 

t  lire 

IX.  Lateral  line  fibers 

IX,  X.  <  Ieneral  \  is<  eral 

IX,  X    To  brani  hial  and 

X.  Lateral   line  and 
general  cutaneous 
fibers 

and  gustatorj 

general  \  isi  eral  mus- 

culat  lire 

12.  There  are  six  pairs  of  cranial  nerves  associated  with  the  medulla  oblongata.  The 
tenth  cranial  or  vagus  nerve  is  one  of  the  largest  and  arises  by  two  series  of  roots.  One 
group  of  rootlets  springs  from  the  dorsolateral  aspect  of  the  medulla  oblongata  near  its 
lower  end,  and  contains  libers  which  are  distributed  through  the  branchial  and  gastro- 
intestinal rami  of  the  vagus,  while  a  large  root,  carrying  fibers  for  the  lateral  line  sense 
organs,  runs  farther  cephalad  and  enters  the  acusticolateral  area.  The  ninth  or  glosso- 
pharyngeal nerve,  the  nerve  of  the  first  branchial  arch,  arises  from  the  medulla  ob- 
longata just  ventral  to  this  root  of  the  vagus.  Since  the  gills,  as  well  as  the  gastro- 
intestinal tract,  are  visceral  organs,  both  the  ninth  and  tenth  nerves  earn-  many  visceral 
fibers.  The  eighth  or  acoustic  nerve  arises  from  the  side  of  the  medulla  opposite  the 
caudal  part  of  the  cerebellum  in  company  with  the  fifth  and  seventh  nerves,  and  ends 
in  the  membranous  labyrinth  of  the  ear.  Like  the  vagus,  the  facial  or  seventh  cranial 
nerve  has,  in  addition  to  its  main  root,  another,  which  runs  further  dorsally  into  the 
acusticolateral  area.  This  root  carries  sensory  fibers  for  the  lateral  line  organs  of  the 
head.  The  sixth  or  abducens  nerve  arises  more  ventrally  at  the  same  level  as  the  eighth. 
The  fifth,  or  trigeminal  nerve,  which  sends  many  branches  to  the  skin  of  the  head,  is 
represented  by  a  large  root  emerging  from  the  medulla  oblongata  in  company  with 
the  seventh.  Some  idea  of  the  peripheral  distribution  of  these  nerves  can  be  gained 
from  a  study  of  Figs.  12  and  13. 

13.  The  floor  of  the  fourth  ventricle  should  now  be  exposed  by  carefully  tearing  away 
the  membranous  roof  of  that  cavity.  The  floor  presents  for  examination  a  -cries  of 
longitudinal  ridges  and  furrows  which  are  of  importance  because  they  mark  the  position 
of  longitudianl  columns  (Figs.  8,  13),  to  each  of  which  a  special  functon  can  be  assigned. 
A  ridge  on  either  side  of  the  midline  represents  the  position  of  the  median  longitudinal 
bundle,  beneath  which  lie  the  nuclei  of  the  third,  fourth,  and  sixth  cranial  nerves. 
Since  these  nerves  supply  somatic  musculature,  the  longitudinal  elevation  marks 
the  position  of  the  somatic  motor  column.     Separated  from  this  ridge  by  a  broad  furrow 


358  THE   NERVOUS    SYSTEM 

is  a  more  prominent  ridge  with  tooth-like  secondary  elevations.  Within  this  second 
ridge  terminate  the  fibers  of  visceral  sensation  and  taste  from  the  seventh,  ninth,  and 
tenth  nerves.  It  is  known  as  the  visceral  lobe  or  visceral  sensory  column.  Beneath  the 
groove  which  separates  these  two  ridges  are  located  the  motor  nuclei  of  the  fifth, 
seventh,  ninth,  and  tenth  cranial  nerves.  These  nuclei  supply  visceral  musculature 
and  constitute  the  visceral  motor  column.  The  dorsal  part  of  the  lateral  wall  of  the  fossa 
forms  another  prominent  ridge,  which  just  caudal  to  the  cerebellum  is  redundant  and 
folded  on  itself  to  form  an  ear-shaped  projection.  This  auricular  fold,  sometimes 
called  the  lobus  linete  lateralis,  and  the  prominent  margin  just  caudal  to  it  belong  to  the 
acusticolateral  area  and  contain  the  centers  for  the  reception  of  impulses  coming  from 
the  ear  (N.  VIII)  and  from  the  sense  organs  of  the  lateral  line  (Nn.  VII  and  X).  Ad- 
jacent to  the  acusticolateral  area  is  a  portion  of  the  medulla  oblongata  which  is  concerned 
with  the  reception  of  sensory  impulses  from  the  skin  which  reach  the  medulla  oblongata 
along  the  fifth  and  tenth  nerves.  The  nuclei  of  the  acusticolateral  and  general  cutane- 
ous areas  together  constitute  the  somatic  afferent  column. 

14.  Locate  these  functional  columns  on  your  specimen.  Note  the  close  relation  of 
the  olfactory  bulb  to  the  nasal  sac.  By  comparison  with  Fig.  13  locate  on  your  speci- 
men the  olfactory  portions  of  the  brain.  What  part  of  the  brain  is  especially  associated 
with  the  eyes? 

15.  Cut  the  nerve  roots  at  some  distance  from  the  brain.  Remove  the  brain, 
being  careful  not  to  injure  the  olfactory  bulbs.  Now  study  the  lateral  and  ventral 
surfaces  of  the  brain  in  order  to  locate  more  accurately  the  points  of  origin  of  the  various 
cranial  nerves  (Fig.  10). 

16.  Now  study  the  parts  of  the  brain  which  belong  to  the  rhombencephalon.  Which 
parts  are  they,  and  what  is  their  relationship  to  each  other?     (Figs.  8,  10  and  p.  26.) 

17.  Study  the  parts  of  the  brain  which  belong  to  the  mesencephalon.  Which 
are  they,  and  what  relationship  do  they  bear  to  each  other?  (Figs.  8,  10  and 
p.  28.) 

18.  In  the  same  way  study  the  parts  belonging  to  the  diencephalon  (Figs..  8,  10 
and  pp.  28,  29).  Make  a  list  of  these  parts.  Tear  awTay  the  membranous  roof  of  the 
third  ventricle  and  examine  that  cavity. 

19.  Note  the  external  form  of  the  telencephalon  and  the  parts  which  compose  it 
(Figs.  8,  10).  Students  working  at  adjacent'  tables  should  cooperate  in  the  work 
which  follows  in  order  that  two  sharks'  brains  may  be  available.  With  a  sharp  razor 
blade  divide  one  in  the  medial  sagittal  plane;  and  with  a  sharp  scalpel  open  up  the 
ventricles  in  the  other  as  indicated  in  Fig.  9.  Study  the  ventricles  of  the  brain  as  they 
are  displayed  in  these  preparations  and  in  Figs.  9  and  11. 

20.  Find  the  velum  transversum  and  the  ridge  produced  by  the  optic  chiasma. 
All  that  part  of  the  brain  which  lies  rostral  to  these  structures  belongs  to  the  telen- 
cephalon. Study  the  telencephalon  in  detail  (Figs.  8-11  and  p.  30).  Of  what  parts 
is  it  composed,  and  what  are  their  relations  to  each  other?  Pay  special  attention  to 
the  several  parts  of  the  telencepha'ic  cavity. 

THE  BRAIN  OF  THE  FETAL  PIG 

21.  Using  a  pig  embryo  of  about  35  mm.,  slice  off  the  skin  and  a  small  amount  of 
the  underlying  tissue  on  either  side  of  the  head  with  a  sharp  razor.     Then  at  one  carefu1 


A    LABORATORY    OUTLINE    OF    NEURO   \\\|n\iv 


stroke  split  the  specimen  lengthwise  in  the  median  plane.     I  his  provides  two  prepara- 
tions for  dissection,  which  should  be  used  l>v  two  students. 


( 'ert  oral  aqueduct 

Lamina  quadrigetnina 
Cerebral  pedunch 
Cerebellu 
Chorioid  plexus  of  fourth  ventru  l< 

Fourth  ventricl 
Medulla  oblongata 


Central  canal  of  spinal 
cord 


Pineal  I 

Third  \> ntrii le 

Hypothalamus 

Thalamus 

Chorioid   plexus   of  lateral 

Vi  iitr'n  h 

I. ati  ral  ventru  le 
■Corpus  striatum 

Lamina  tcrminalis 

Rhinent  <  plialon 

Hypophysis 

Tongue 


Fig.  258. — Medial  sagittal  section  of  the  head  of  a  35  nun.  pig  embryo.      (Redrawn  from  Prentiss- 

Arey.) 

22.  First  study  the  medial  section  of  the  brain,  noting  the  five  divisions  of  tin 
brain,  the  ventricles,  and  the  relation  of  the  cerebral  hemispheres  to  other  parts  of  the 


Semilunar  ganglion  N.  V 
Mi  sencepkalon 

Cerebellum 


Hypothalamus 


Geniculate  gang.  X.  I'll 

Ganglion  N.  VIII 

Medulla  oblongata 

Jugular  gang.  N.  X 

Gang,  of  Froriep 

Gang.  X.  cerv.  I 

Accessory  nerve 

Hypoglossal  nerve 

Ganglion  nodosum  X.  X 

Gang.  X.  cerv.  V 


Cerebral  hemisphere 

X.  1'.  ophthalmic  X. 

Rhinenccplialon 
X .  opticus 
X .  1'.  maxillary  X . 
X .  V .  mandibular  X. 


Chorda  tympani 
Facial  X . 


Fig.   259-  Dissection  of  the  head  of  a  35  mm.  pig  embryo.     Lateral  view.      (Redrawn  from 

Prentiss-  Arey.) 


brain  (Fig.  258.     See  also  Figs.  16,  17  and  pp.  32-36).     Of  what  three  parts  is  the 
cerebral  hemisphere  composed?     Locate  each  of  the  subdivisions  of  the  dieneephalon. 


360  THE   NERVOUS   SYSTEM 

To  which  part  does  the  pineal  body  belong?     The  hypophysis?     Locate  the  quadri- 
geminal  lamina,  cerebral  peduncle,  cerebellum,  and  medulla  oblongata. 

23.  Now  turn  the  specimen  over  and  carefully  dissect  away  what  remains  of  the 
skin  and  mesodermal  tissues  so  as  to  expose  the  brain  and  cranial  nerves  from  the  lateral 
side.     Identify  all  the  parts  labeled  in  Fig.  259. 

GENERAL  TOPOGRAPHY  OF  THE  BRAIN 

24.  The  adult  mammalian  brain  should  now  be  compared  with  that  of  the  shark 
and  with  that  of  the  fetal  pig.  If  two  sheeps'  brains  are  available,  one  should  be  divided 
into  lateral  halves  by  a  cut  made  exactly  1  mm.  to  the  left  of  the  median  sagittal  plane. 
Use  a  long,  thin  brain  knife  and  make  the  cut  with  a  single  sweep.  Put  away  the  right 
half  for  future  study.  On  the  left  half  and  on  the  intact  brain  identify  all  of  the  chief 
divisions  of  the  brain,  determine  their  embryologic  derivation,  and  compare  them 
with  similar  parts  in  the  brains  of  the  shark  and  fetal  pig.  (See  the  table  on  p.  36, 
pp.  113-116,  and  Figs.  82-84.) 

25-  By  a  study  of  the  medial  aspect  of  the  left  half  of  the  brain  ascertain  what 
relations  the  various  subdivisions  bear  to  each  other.  (See  Fig.  84  and  pp.  116-118.) 
Note  the  difference  in  color  between  the  cortex  and  the  white  center  of  the  cerebellum. 
By  tearing  away  the  cerebellum  a  little  at  a  time  make  a  dissection  of  the  cerebellar 
peduncles  on  this  half  of  the  brain  (Figs.  87,  91).  Scrape  away  the  superficial  gray 
matter  from  the  rostral  end  of  the  left  hemisphere  and  uncover  the  white  substance 
beneath.  The  superficial  gray  matter  is  known  as  the  cerebral  cortex  and  this  covers 
the  white  center  of  the  cerebral  hemisphere. 

NEUROLOGIC  STAINS 

26.  Some  knowledge  of  how  various  stains  act  on  the  nervous  tissues  is  essential 
for  an  understanding  of  the  special  preparations  which  are  to  be  studied.  The  technic 
involved  in  preparing  such  material  is  described  in  books  devoted  to  technical  methods 
(Hardesty,  1902;  Guyer,  1917). 

27.  Osmic  Acid. — Small  nerves  may  be  fixed  in  osmic  acid.  This  stains  the  myelin 
sheaths  black.     Why?     Axons  remain  unstained. 

28.  The  Weigert  or  Pal-Weigert  Method. — When  a  portion  of  the  brain  or  spinal 
cord  has  been  treated  for  several  weeks  with  a  solution  containing  potassium  bichromate 
(Miiller's  fluid)  the  myelin  sheaths  acquire  a  special  affinity  for  hematoxylin,  by 
virtue  of  which  they  become  deep  blue  in  color  when  stained  by  this  method.  Axons, 
nerve-cells,  and  all  other  tissue  elements  remain  colorless  unless  the  preparation  has 
been  counterstained.  The  method  is  adapted  for  the  study  of  the  development  and 
extent  of  myelination  and  for  tracing  myelinated  fiber  tracts.  This  method  may  also 
be  used  for  a  study  of  degenerated  fiber  tracts,  which  remain  colorless  in  preparations 
in  which  the  normal  fiber  tracts  are  well  stained. 

29.  The  Marchi  method  is  a  differential  stain  for  degenerating  fibers.  These 
contain  droplets  of  chemically  altered  myelin.  The  tissue  is  fixed  in  a  solution  contain- 
ing potassium  bichromate  (Miiller's  fluid).  This  treatment  prevents  the  normal 
myelinated  fibers  from  staining  with  osmic  acid,  but  does  not  prevent  the  droplets  of 
chemically  altered  myelin  in  the  degenerated  fiber  from  being  stained  black  by  this 


A    LABORATORY    OTJ  I  LINE    OF    \i  I  RO   \\  \  [OlfY 

reagent.     In  a  section  prepared  by  this  method  the  normal  myelinated  fibers  are  light 
yellow,  while  the  degenerated  fibers  are  represented  by  rows  of  bla<  I.  dots. 

30.  The  newer  silver  stains,  including  the  Cajal  method  and  the  pyridin-silver  technic, 
depend  upon  the  special  affinity  for  silver  nitrate  possessed  by  nerve-cells  and  their 
processes.  After  treatment  with  silver  nitrate  tin-  tissue  is  transferred  t<»  a  solution 
of  pyrogallic  acid  or  hydroquinon  which  reduces  the  silver  in  the  neurons  to  a  metallic 
state.  Nerve-cells  and  their  processes  are  stained  yellow  or  brown  by  these  methods. 
Myelin  sheaths  remain  unstained.  The  axis-cylinders  of  the  myelinated  ill  i  i 
light  yellow,  the  unmyelinated  axons  are  dark  brown  or  black.  The  neurofibri 
stained  somewhat  more  darkly  than  other  parts  of  the  cytoplasm. 

31.  The  Golgi  method  furnishes  preparations  which  demonstrate  the  external 
form  of  the  neurons,  and  make  it  possible  to  trace  individual  axons  and  dendrit< 
considerable  distances.  The  method  also  stains  neuroglia.  It  is  selective  and  rather 
uncertain  in  its  results,  since  only  a  small  proportion  of  the  nerve-cells  are  impregnated 
in  any  preparation.  The  stain  is  due  to  the  impregnation  of  the  nerve-cells  and  their 
processes  with  silver. 

32.  The  best  stains  for  demonstrating  the  tigroid  masses  or  Nis>l  bodies  are 
toluidin  blue  and  Nissl's  methylene-blue.  Both  are  basic  dyes;  and  in  properly  fixed 
nervous  tissue  they  color  the  tigroid  masses  as  well  as  the  nuclear  chromatin  of  nerve- 
cells  blue. 

THE  PERIPHERAL  NERVOUS  SYSTEM 

33.  The  Spinal  Ganglia. — Study  a  longitudinal  section  through  a  spinal  nerve  and 
its  roots,  including  the  spinal  ganglion,  stained  by  the  pyridin-silver  method.  How 
are  myelinated  and  unmyelinated  axons  stained  by  this  method?  What  kinds  of  cells 
do  you  find?  Study  their  axons.  (See  Figs.  39,  40  and  pp.  62-66.)  Look  for  the 
bifurcation  of  the  myelinated  and  unmyelinated  fibers.  Note  the  differences  in 
composition  of  the  ventral  and  dorsal  roots.  What  becomes  of  the  various  kinds  of 
fibers  when  traced  peripherally?  When  traced  toward  the  spinal  cord?  What  is  the 
origin  of  the  unmyelinated  fibers? 

34.  Study  the  vagus  nerve  of  the  dog  in  osmic  acid  and  pyridin-silver  preparations. 
How  are  the  various  kinds  of  nerve-fibers  stained  in  each?  How  does  the  structure  of 
the  vagus  differ  from  that  of  a  spinal  nerve? 

35.  Study  the  cervical  portion  of  the  sympathetic  trunk,  which  in  the  dog  lies  in  a 
common  sheath  with  the  vagus.  Of  what  kind  of  fibers  is  it  composed?  What  is  the 
origin  and  termination  of  these  fibers?     (See  pp.  345-347.) 

36.  Study  the  pyridin-silver  preparation  from  the  superior  cervical  sympathetic 
ganglion.  What  is  the  source  of  the  fine  black  fibers,  and  where  do  they  end?  Study 
the  ganglion  cells.     What  becomes  of  their  axons?     (See  Figs.  251,  253  and  pp.  341-544.) 

THE  SPINAL  CORD 

37.  Review  the  development  and  gross  anatomy  of  the  spinal  cord  (p.  42  and  pp. 
73-78).  Examine  the  demonstration  preparations  of  the  vertebral  column,  showing 
the  spinal  cord  exposed  from  the  dorsal  side.  In  these  preparations  study  the  meninges 
and  ligamentum  denticulatum,  as  well  as  the  shape  and  size  of  the  spinal  cord.     Note 


362  THE    NERVOUS    SYSTEM 

the  level  of  the  termination  of  the  spinal  cord,  the  level  of  the  origin  of  the  various 
nerve  roots  and  of  their  exit  from  the  vertebral  canal,  and  the  level  of  the  various  seg- 
ments of  the  cord  with  reference  to  the  vertebrae.  Note  the  filum  terminale  and  the 
Cauda  equina.  From  your  text-books  of  anatomy  study  the  meninges  and  blood- 
supply  of  the  cord. 

38.  The  Spina!  ( 'ord  in  Section—  Examine  the  Pal-Weigert  sections  of  the  cervical, 
thoracic,  lumbar,  and  sacral  regions,  and  from  them  reconstruct  a  mental  picture  of  the 
topography  of  the  entire  cord.  How  does  it  vary  in  shape  and  size  at  the  different 
levels?  Identify  all  the  fissures,  sulci,  septa,  funiculi,  gray  columns,  commissures  and 
nerve  roots,  the  reticular  formation,  the  substantia  gelatinosa  and  the  caput,  cervix, 
and  apex  of  the  posterior  gray  column.     (See  pp.  78-84.) 

39.  The  Microscopic  Anatomy  of  the  Spinal  Cord. — Study  all  of  the  histologic 
preparations  of  the  spinal  cord  which  have  been  furnished  you.  (See  pp.  85-90.) 
Study  the  neuroglia  in  Golgi  preparations.  Study  the  pia  mater,  septa,  blood-vessels, 
and  ependyma  in  hematoxylin  and  eosin  preparations.  Study  the  nerve-cells  in  Nissl, 
Golgi,  and  silver  preparations.  Study  the  myelinated  fibers  in  Weigert  preparations 
and  both  the  myelinated  and  unmyelinated  fibers  in  the  silver  preparations.  Note 
the  arrangement  of  each  of  these  histologic  elements  and  be  sure  that  you  understand 
the  relations  which  they  bear  to  each  other. 

40.  Draw  in  outline,  ventral  side  down,  each  of  four  Pal-Weigert  sections  taken, 
respectively,  through  the  cervical,  thoracic,  lumbar,  and  sacral  regions  of  the  human 
spinal  cord.  Make  the  outlines  very  accurate  in  shape  and  size,  with  an  enlargement 
of  8  times.  Put  in  the  outline  of  the  gray  columns,  the  central  canal,  and  the  substantia 
gelatinosa  Rolandi.  Put  each  outline  on  a  separate  sheet  and  do  not  ink  the  drawings 
at  present. 

41.  Identify  the  various  cell  columns  in  the  gray  matter  and  note  how  they  vary 
in  the  different  levels  of  the  cord  (Nissl  or  counterstained  Weigert  preparations). 
(See  pp.  89,  90  and  Fig.  65.)  Indicate  these  cell  groups  in  their  proper  places  in  the 
four  outline  sketches  of  the  spinal  cord.  What  becomes  of  the  axons  arising  from 
each  group  of  cells?  Why  are  the  anterolateral  and  posterolateral  cell  groups  seen 
only  in  the  regions  associated  with  the  brachial  and  lumbosacral  plexuses?  The 
intermediolateral  column  only  in  the  thoracic  and  highest  lumbar  segments?  Why  is 
the  gray  matter  most  abundant  in  the  region  of  the  intumescentiae  and  the  white  matter 
most  abundant  at  the  upper  end  of  the  spinal  cord? 

42.  What  elements  are  concerned  in  spinal  reflexes?     (See  pp.  91-94.) 

43.  What  connections  do  the  fibers  of  the  spinal  nerves  establish  in  the  spinal  cord? 
What  is  the  origin  and  the  peripheral  termination  of  the  somatic  efferent  fibers,  of  the 
visceral  efferent  fibers,  of  the  somatic  afferent  fibers,  and  of  the  visceral  afferent  fibers 
of  the  spinal  nerves?  (See  pp.  60-63  and  Fig.  37.)  What  are  the  proprioceptive 
and  exteroceptive  fibers,  and  in  what  peripheral  structures  do  they  end?  (See  pp. 
66-72.) 

44.  In  a  pyridin-silver  preparation  of  the  cervical  spinal  cord  of  a  cat  note  that  as 
the  dorsal  root  enters  the  cord  the  unmyelinated  fibers  run  through  the  lateral  division 
of  the  root  into  the  dorsolateral  fasciculus  (Fig.  72).  The  medial  division  of  the  root 
is  formed  of  myelinated  fibers  which  enter  the  posterior  funiculus.  Read  about  the 
intramedullay  course  of  these  fibers  (pp.  95-98)., 


A    LABORA  ruin     01    I  LINE    OF    \l  l  R(  I    \\  \  in\i\ 

45.  The  fiber  tracts,  of  which  the  white  substance  is  compo  ed,  cannot  be  distin- 
guished in  the  normal  adult  cord.    They  can  be  recognized  from  dim  n 
of  their  rnyelination  in  fetal  cords  (p.  112  and  Fig.  79)  and  in  preparation 
degeneration  resulting  from  disease  or  injury  in  various  parts  oi  the  nervou 
(p.  LOS;  Figs.  75,  76).     From  such  preparations  as  arc  available  for  this  purpose  and 
from  your  reading  (pp.  95  112)  form  a  clear  conception  of  the  origin,  course,  and  ter- 
mination of  each  of  the  fiber  traits. 

4(>.    Indicate   the   location   of  each   of   these   tracts   in    the  outline  drawing  of   the 

cervical  portion  of  the  spinal  cord,  entering  the  ascending  tracts  and  the  ventral  i  orti<  o- 
spinal  tract  on  the  right  side,  and  all  of  the  descending  tra<  tsex<  epl  the  ventral  cortico- 
spinal tract  on  the  left  side.    Why  should  the  ventral  and  lateral  corticospinal  tracts 

be  indicated  on  opposite  sides  of  the  cord?     Wax  crayons  should  be  used  to  give  the 
several  tracts  a  differential  coloring.    Use  the  Eollowing  color  scheme: 
Somatic  afferent  tracts: 
Proprioceptive — yellow. 
Exteroceptive — blue. 
Somatic  motor  tracts: 

Corticospinal  tracts — red. 
Rubrospinal  tract — brown. 
All  other  tracts— black. 

47.  The  fasciculus  cuneatus  and  fasciculus  gracilis  should  be  colored  yellow  and 
then  clotted  over  with  blue  to  indicate  that  while  the  proprioceptive  fibers  predominate, 
there  are  also  some  exteroceptive  libers  in  these  tracts. 

THE  BRAIN  STEM 

48.  Now  take  the  human  brain  and  identify  all  of  its  principal  divisions.  Dissect 
out  the  arterial  circle  of  }]'illi.s,  and  identify  the  branches  of  the  internal  carotid,  ver- 
tebral, and  basilar  arteries.  Read  about  the  blood-supply  and  meninges  of  the  brain 
in  your  text-book  of  anatomy.     Identify  all  of  the  cranial  nerves  (Fig.  86). 

49.  Examine  again  the  cerebellar  peduncles  in  the  three  specimens  of  the  sheep- 
brain  (Figs.  87,  91).  Now  remove  the  cerebellum  from  the  previously  intact  sheep- 
brain.  Cut  through  the  peduncles  on  both  sides  of  the  brain  as  far  as  possible  from 
the  pons  and  medulla,  sacrificing  the  cerebellum  to  some  extent  in  order  to  leave  as 
much  of  the  peduncles  as  possible  attached  to  the  brain  stem.  Be  careful  not  to  damage 
the  anterior  medullary  velum  and  the  tela  chorioidea  which  lie  under  cover  of  the 
cerebellum  (Fig.  84).     In  the  same  way  remove  the  cerebellum  from  the  human  brain. 

50.  Study  the  roof  of  the  fourth  ventricle  in  both  the  human  and  the  sheep's  brain 
(pp.  128,  129  and  Figs.  84,  90,  154).  Examine  the  chorioid  plexus  of  the  fourth  ven- 
tricle. Note  the  line  of  attachment  of  the  tela  chorioidea.  Tear  this  membrane  away. 
The  torn  edge  which  remains  attached  to  the  medulla  is  the  taenia  of  the  fourth  ventricle 
(Figs.  89,  90).  Study  the  attachments  of  the  anterior  medullary  velum.  The  decus- 
sation of  the  trochlear  nerve  within  the  velum  can  easily  be  seen  in  the  sheep.  Remove 
this  membrane.     The  floor  of  the  fourth  ventricle  is  now  fully  exposed. 

51.  Remove  the  pia  mater  from  the  brain  stem,  carefully  cutting  around  the  roots 
of  the  cranial  nerves  with  a  sharp-pointed  knife  to  prevent  these  nerves  being  torn 
away  from  the  brain  when  this  membrane  is  removed. 


364  THE    NERVOUS    SYSTEM 

52.  Carefully  examine  the  medulla,  pons,  floor  of  the  fourth  ventricle,  and  the  mesen- 
cephalon, observing  all  the  details  mentioned  on  pp.  118-131  and  illustrated  in  Figs. 
84,  86-89,  91. 

53.  Take  selected  transverse  sections  through  the  human  brain  stem  and,  by  com- 
parison with  the  gross  specimen,  determine  the  level  of  each  section. 

54.  Draw  in  outline  each  of  these  transverse  sections  through  the  brain  stem. 
Put  each  drawing  on  a  separate  page,  ventral  side  down,  with  the  transverse  diameter 
corresponding  to  the  longer  dimension  of  the  paper.  Study  each  preparation  in  detail 
and  identify  all  of  the  parts,  indicating  them  lightly  in  pencil.  Do  not  label  the  draw- 
ings at  this  time.  Make  sure  that  all  proportions  are  correct.  The  sections  through 
the  medulla  should  be  enlarged  eight  diameters,  those  through  the  pons  and  mesen- 
cephalon four  diameters. 

55.  Section  Through  the  Decussation  of  the  Pyramids. — Keep  in  mind  the  tracts 
which  extend  into  the  brain  from  the  spinal  cord  and  note  the  changes  in  their  form 
and  position.  Identify  the  decussation  of  the  pyramids,  the  nucleus  gracilis  and  nucleus 
cuneatus,  the  spinal  root  of  the  trigeminal  nerve  and  its  nucleus,  the  reticular  formation. 
Note  the  change  in  the  form  of  the  gray  substance  (pp.  132-137;  Figs.  94,  95,  98). 

56.  Section  Through  the  Decussation  of  the  Lemniscus. — Note  the  rapid  change  in 
the  form  of  the  gray  matter.  Identify  the  internal  and  external  arcuate  fibers,  the 
decussation  of  the  lemniscus  and  the  beginning  of  the  medial  lemniscus,  as  well  as  the 
structures  continued  up  from  the  preceding  level  (Figs.  96,  99;  pp.  137-139). 

57.  Section  Through  the  Olive  and  the  Hypoglossal  Nucleus. — At  this  level  the  central 
canal  opens  out  into  the  fourth  ventricle.  The  posterior  funiculi  and  their  nuclei  are 
disappearing  or  have  disappeared.  The  dorsal  spinocerebellar  tract  lies  lateral  to  the 
spinal  tract  of  the  trigeminal  nerve  and  is  directed  obliquely  backward  toward  the 
restiform  body.  Identify,  in  addition  to  those  structures  which  are  continued  from 
the  preceding  level,  the  inferior  olivary  nucleus  with  the  olivocerebellar  fibers,  the 
dorsal  and  medial  accessory  olivary  nuclei,  the  external  arcuate  fibers,  the  nucleus  and 
fibers  of  the  hypoglossal  nerve,  the  dorsal  motor  nucleus  of  the  vagus,  the  tractus 
solitarius  and  its  nucleus,  the  nucleus  ambiguus  and  the  lateral  reticular  nucleus  (Figs. 
97,  101;  pp.  139-142). 

58.  Section  Through  the  Restiform  Body. — The  restiform  body  and  the  spinal  tract 
of  the  fifth  nerve  are  conspicuous  in  the  dorsolateral  part  of  the  section.  In  the  floor 
of  the  fourth  ventricle  locate  the  nucleus  of  the  hypoglossal  nerve,  the  dorsal  motor 
nucleus  of  the  vagus,  the  medial  and  the  spinal  vestibular  nuclei.  The  spinal  tract  of 
the  fifth  nerve  and  its  nucleus  are  deeply  situated  ventral  to  the  restiform  body  and 
broken  up  by  the  olivocerebellar  fibers  (Fig.  103;  pp.  143-146). 

59.  Section  Through  the  Lower  Margin  of  the  Tons. — Identify  such  portions  of  the 
pons,  brachium  pontis,  and  cerebellum  as  are  contained  in  the  section.  Dorsolateral 
to  the  restiform  body  is  the  dorsal  cochlear  nucleus,  and  ventrolateral  to  it  the  ventral 
cochlear  nucleus.  Identify  the  stria?  medullares  and  the  beginning  of  the  trapezoid 
body,  also  the  medial  and  lateral  vestibular  nuclei  (Fig.  107;  pp.  149-152). 

60.  Section  'Through  the  Facial  Colliculus. — Differentiate  between  the  ventral  and 
the  dorsal  portions  of  the  pons,  and  in  the  ventral  portion  identify  the  longitudinal 
fasciculi,  transverse  fibers,  and  the  nuclei  pontis  (pp.  147-149).  In  the  dorsal  part 
identify  the  nuclei  and  root  fibers  of  the  sixth  and  seventh  nerves  including  the  genu 


A    LABORATORY    OUTLINE    01    NEURO   \\\n>\i\  ,()- 

of  the  seventh  nerve.     Locate  the  spinal  trad  of  the  fifth  nerve  and  it    nucleus  the 
trapezoid  body,  and  superior  olivary  nucleus  (Fig.  108;  pp.  151   154 

61.  Section  Through  the  Middle  of  the  Pons  Showing  the  Motor  and  Main  Sen 
Nuclei  of  the  Fifth  Nerve.     In  addition  to  these  nuclei  note  the  beginning  of  the  mi 
cephalic  root  of  the  fifth  nerve.    The  brachium  conjunctivum  makes  it-  appearance 
in  the  dorsal  part  of  the  section  (Fig.  110;  pp.  154   157). 

()_'.  Section  Through  the  Inferior  Colliculus.  Identify  the  basis  pedum  uli,  substantia 
nigra,  medial  and  lateral  lemnisci,  cerebral  aqueduct,  central  gray  matter,  mesence- 
phalic root  of  the  fifth  nerve,  fasciculus  longitudinalis  medialis,  nucleus  of  the  tnw  blear 
nerve,  and  the  decussation  of  the  brachium  conjunctivum  (Figs.  1 13,  1 14;  pp.  158,  165). 

63.  Section  Through  the  Superior  Colliculus.  —Identify,  in  addition  to  the  3tru<  tun- 
continued  upward  from  lower  levels,  the  red  nucleus,  the  nucleus  of  the  third  nerve, 
and  the  root  libers  of  that  nerve,  the  ventral  and  dorsal  tegmental  decussations,  the 
inferior  quadrigeminal  brachium,  and  the  medial  geniculate  body  (Fig.  116;  pp.  160, 
167). 

THE  CEREBELLUM 

64.  Compare  the  human  cerebellum  with  that  of  the  shaik  and  the  sheep.  How 
is  its  size  related  to  the  size  of  the  pons  and  to  the  extent  of  the  cerebral  cortex? 

65.  On  both  the  human  and  sheep's  cerebellum  identify  the  vermis,  hemispheres, 
and  divided  peduncles  (Figs.  138,  139,  143-145).  In  the  medial  sagittal  section  of 
the  sheep's  brain  identify  the  white  medullary  body  of  the  cerebellum,  the  arbor 
vita?,  cerebellar  cortex,  folia,  and  sulci  (Fig.  84;  pp.  196-199). 

66.  Study  the  morphology  of  the  cerebellum  in  the  sheep  (Figs.  143-145).  Lo- 
cate these  same  fundamental  subdivisions  in  the  human  cerebellum  (Figs.  146,  147). 
What  functions  have  recently  been  assigned  to  each  of  these  subdivisions?  (See 
pp.  199-203.) 

67.  Divide  the  human  cerebellum  in  the  median  plane.  Cut  the  right  half  into 
horizontal  sections  and  the  left  into  sagittal  sections  and  study  the  medullary  center 
and  nuclei  of  the  cerebellum  (Figs.  140,  141,  148;  pp.  199,  203). 

68.  Study  the  histologic  sections  of  the  cerebellar  cortex  and  master  the  details 
of  its  structure  (Figs.  150,  151;  pp.  206-210). 

FUNCTIONAL  ANALYSIS  OF  THE  BRAIN  STEM 

69.  Review  the  sections  of  the  brain  stem  as  directed  in  the  following  paragraphs, 
paying  special  attention  to  the  functional  significance  of  the  various  nuclei  and  fiber 
tracts  as  far  as  they  can  be  followed  in  the  series  of  sections.  In  general,  the  afferent 
tracts  and  nuclei  should  be  entered  in  color  on  the  right  side  of  the  drawings  already 
made,  and  the  efferent  tracts  and  nuclei  on  the  left  side.  But  this  order  must  be  re- 
versed in  certain  cases  to  allow  for  the  decussation  of  the  tracts.  Label  the  various 
tracts  and  nuclei.     Use  the  following  color  scheme: 

Somatic  afferent: 

Exteroceptive — blue. 

Proprioceptive — yellow. 
Visceral  afferent — orange. 
Visceral  efferent — purple. 


366  If]].    NERV01  -    SYSTEM 

Somatic  efferent — red. 

All  1  erebellar  connections  not  strictly  proprioceptive — brown. 

Other  tracts — black. 

PROPRIOCEPTIVE  PATHS  AND  CENTERS     pp.  311-315) 

70.  The-  cerebellum  is  the  chief  proprioceptive  correlation  center,  and  the  restiform 
body  consists  for  the  most  part  of  proprioceptive  afferent  path-  (Fiji.  235  .  Note  its 
shape,  position,  and  connections  in  all  the  gross  specimens.  In  the  left  lateral  half  of 
tin-  sheep's  brain  follow  it  caudally  by  dissection,  separating  it  from  the  other  peduncles. 
Cut  and  reflect  the  dorsal  cochlear  nucleus  of  the  eighth  nerve.  Trace  the  restiform 
body  backward  and  note  the  accession  of  external  arcuate  fibers.  At  the  level  of  the 
inferior  olive  it  receives  the  dorsal  spinocerebellar  tract.  Trace  this  by  dissection  from 
the  restiform  body  obliquely  across  the  upper  end  of  the  tuberculum  cinereum  and 
then  caudally  along  the  ventral  border  of  this  elevation  to  the  spinal  cord.  (See  Figs. 
87,  88,  104;  pp.  143,  205.) 

71.  Now  take  the  sections  of  the  medulla,  locate  the  dorsal  spinocerebellar  tract 
in  each,  and  indicate  its  position  in  yellow  on  the  right  side  of  your  outlines  (p.  144  . 
Locate  the  external  arcute  fibers  (p.  139).  From  where  do  they  come  and  where  do  they 
go?  Draw  in  yellow  those  belonging  to  the  right  peduncle.  Locate  in  your  sections 
the  olivocerebellar  tract,  and  with  brown  indicate  in  your  outline  the  fibers  running  into 
the  right  peduncle  (Fig.  103). 

72.  From  your  texts  ascertain  the  course  of  the  ventral  spinocerebellar  tract  and 
indicate  its  position  in  yellow  on  the  right  side  of  the  outlines  (Fig.  149;  p.  157;. 

73.  Proprioceptive  Path  to  the  Cerebral  Cortex. — Indicate  in  yellow  the  terminal 
portion  of  the  right  dorsal  funiculi,  and  with  yellow  stipple  the  right  nucleus  gracilis 
and  nucleus  cuneatus  (Figs.  98,  99).  Study  the  internal  arcuate  fibers  and  the  medial 
lemniscus,  drawing  the  internal  arcuate  fibers  from  right  to  left  and  the  medial  lemniscus 
on  the  left  side  (yellow).  Where  do  the  fibers  of  the  medial  lemniscus  terminate? 
What  is  the  source  and  what  the  destination  of  the  impulses  which  they  carry?  (See 
Figs.  101,  103,  107,  108,  110,  114,  116,  235  and  pp.  138,  312.) 

74.  Locate  the  vestibular  nuclei  and  indicate  them  with  yellow  stipple  on  the  right 
side  of  the  outlines  (Figs.  101,  103,  107,  108).  Locate  the  vestibulocerebellar  tract 
(pp.  151,  188;  Fig.  136). 

EXTEROCEPTIVE  PATHS  AND  CENTERS  (pp.  302-310) 

75.  The  Cochlear  Nerve  and  its  Connections. — On  the  sheep's  brain  note  the  two 
divisions  of  the  acoustic  nerve  as  well  as  the  ventral  and  dorsal  cochlear  nuclei  and  the 
trapezoid  body  (Fig.  K7).  Examine  the  cochlear  nuclei  and  the  striae  medullares  in  the 
human  brain  (Fig.  89).  Locate  the  lateral  lemniscus  where  it  forms  a  flat  band  of 
fibers  directed  rostrally  and  dorsally  upon  the  lateral  surface  of  the  mesencephalon. 
It  occupies  a  triangular  sp  ice  dorsal  to  the  basis  pedunculi  and  rostral  to  the  pons  and 
is  superficial  to  the  brachium  conjunctivum  (Fig.  88). 

76.  Now  take  the  section  through  the  lower  border  of  the  pons  and  study  the 
cochlear  nuclei,  the  s trice  medullares,  and  the  beginning  of  the  trapezoid  bod\  (Fig.  107). 
In  the  section  through  the  facial  colliculus  study  the  trapezoid  body  and  the  superior 


\    I  \r.<  IB  \  lnkV    01   I  LINE    "I     Ml  Ri  I    VNAT01TV 

olivary  nuclei  (Pig.  108).  In  the  section  through  the  middle  of  the  pons  identify  the 
lateral  lemniscus.  Trace  this  trad  to  the  inferior  colliculu^s  I  ig.  11  I  and  through  the 
inferior  quadrigeminal  brachium  to  the  medial  geniculate  bod)  Figs.  114,  116  Color 
these  centra]  connections  of  the  co<  blear  nerve  blue,  indicating  the  co<  blear  nuclei  on 
the  right  side  and  the  lateral  lemniscus  on  the  lefl  '  I  ig.  134;  pp.  1  19,  1 5 

77.  Dissection  of  the  spinal  tract  of  the  fifth  inn,.    On  the  lefl  half  of  the  - 
brain  locate  the  fifth  nerve  and  tear  away  the  transverse  fibers  of  the  pons  caudal  to 
that  nerve  until  the  longitudinal  fibers  of  its  spinal  tract  arc  exposed.     By  carefully 
scraping  away  the  structures  superficial  to  this  tract  follow  it  to  the  lower  end  of  the 
medulla. 

78.  Locate  the  sensory  nuclei  of  the  fifth  nerve  in  your  sections  and  indicate  them  with 
colored  stipple  on  the  right  side  of  your  drawing  (pp.  154,  182;  Fig.  131):  the  m 
cephalic  nucleus,  yellow  (Fig.  114);  the  main  sensory  nucleus,  blue  (Fig.  11<)  ;  the 
nucleus  of  the  spinal  tract,  blue  (Figs.  98,  99,  101,  103,  107,  108).  At  the  same  time 
color  the  spinal  tract  of  the  right  side  blue.  What  becomes  of  the  fibers  which  arise 
from  the  cells  of  the  main  sensory  and  the  spinal  nuclei  of  the  trigeminal  nerve? 

pp.  183,  507:  Fig.  232.) 

79.  From  the  text  ascertain  the  course  of  the  spinothalamic  tract  and  trace  it  up 
through  the  brain  stem  (Figs.  105,  230,  231,  234).  Where  do  these  fibers  come  from, 
and  where  do  they  end?  What  kind  of  sensations  do  they  mediate?  Enter  it  in  blue 
on  the  right  side  of  your  drawings.     (See  pp.  101,  102,  145,  M)^J 

VISCERAL  AFFERENT  PATHS  AND  CENTERS 

80.  Identify  the  tr actus  solitarius  and  its  nucleus  (Figs.  101,  103,  120).  What  is 
the  origin,  termination,  and  function  of  the  fibers  constituting  this  tract?  (See  pp. 
180,  181.)  Indicate  the  tract  with  orange  and  the  nucleus  with  orange  stipple  on  the 
right  side  of  your  drawing. 

VISCERAL  MOTOR  CENTERS 

81.  In  the  sections  of  the  brain  stem  identify  the  dorsal  motor  nucleus  of  the  vagus 
(Figs.  101,  103)  and  the  following  special  visceral  motor  nuclei:  the  nucleus  ambiguus 
(Figs.  101,  103),  the  motor  nucleus  of  the  fifth  (Fig.  110),  and  the  motor  nucleus  of  the 
seventh  nerve  (Fig.  108).  Stipple  these  nuclei  purple  on  the  left  side.  How  are  visceral 
afferent  and  efferent  elements  connected  to  form  visceral  reflex  arcs?     (See  pp.  1 74   178.) 

SOMATIC  MOTOR  TRACTS  AND  CENTERS 

82.  The  Corticospinal  and  Corticopontine  Tracts. — From  the  cerebral  cortex  the 
fibers  of  the  pyramidal  tract  run  through  the  internal  capsule  and  brain  stem  to  the 
somatic  motor  and  special  visceral  motor  nuclei  of  the  cranial  nerves  and  to  the  anterior 
gray  column  of  the  spinal  cord.  Along  with  these  it  will  be  convenient  to  study  the 
cortico-ponto-cerebellar  pathway.  Take  the  left  lateral  half  of  the  sheep's  brain  and, 
being  careful  not  to  injure  the  optic  tract  and  optic  radiation,  follow  the  fibers  ot  the 
basis  pedunculi  by  dissection  through  the  internal  capsule  to  the  cerebral  cortex 
260).  Now  tear  away  the  transverse  fibers  of  the  pons  a  few  at  a  time  and  follow  them 
by  dissection  into  the  brachium  pontis.  Observe  that  some  of  the  fibers  oi  the  basis 
pedunculi  end  in  the  pons  (corticopontine  fibers)  and  that  others  (corticospinal  i. 


368  THE    NERVOUS   SYSTEM 

can  be  traced  through  the  pons  into  the  pyramid  of  the  medulla.  Carrying  the  dis- 
section caudally,  observe  the  decussation  in  the  lower  end  of  the  medulla. 

83.  Examine  again  the  series  of  sections  through  the  brain  stem  and  color  the 
corticospinal  tract  red  on  the  right  side  of  your  drawings.  Draw  the  fibers  from  right 
to  left  in  the  decussation  (Fig.  237;  pp.  136,  317). 

N4.  With  red  stipple  indicate  the  somatic  motor  nuclei  on  the  left  side  of  your  draw- 
ings.    Which  nuclei  are  they?     (See  pp.  170-173.) 

CEREBELLAR  CONNECTIONS 

85.  The  inferior  peduncle  has  already  been  studied  and  the  cortico-ponto-cerebellar 
path  has  been  dissected.  Review  this  path  in  your  sections.  Color  the  corticopontine 
tracts  of  the  left  side  brown  (  Fig.  117).  Indicate  the  nuclei  pontis  of  the  left  side  by 
brown  stipple.  Draw  the  transverse  fibers  of  the  pons  from  the  left  nuclei  pontis  to 
the  right  brachium  pontis  (Fig.  106;  pp.  147-149). 

86.  In  the  left  lateral  half  of  the  sheep's  brain  follow  the  brachium  conjunctivum 
by  dissection  into  the  tegmentum  of  the  mesencephalon  and  note  its  decussation 
beneath  the  inferior  colliculus.  In  your  sections  trace  it  rostrally,  noting  its  decus- 
sation and  termination  (Figs.  110,  112,  114-116).  Indicate  it  in  brown  on  your 
drawings,  beginning  on  the  right  side  and  tracing  it  through  the  decussation  to  the  left 
red  nucleus.     Stipple  both  red  nuclei  with  brown.     (See  pp.  159,  326.) 

87.  The  Rubrospinal  Tract. — Trace  the  rubrospinal  tract  from  the  red  nucleus 
through  the  ventral  tegmental  decussation  (Fig.  116)  and  the  reticular  formation  of  the 
brain  stem.  In  the  reticular  formation  it  occupies  a  position  ventromedial  to  the 
nucleus  of  the  spinal  root  of  the  trigeminal  nerve  (Figs.  115,  234;  pp.  161,  326).  Color 
it  brown  on  the  left  side  of  your  drawings. 

THE  RETICULAR  FORMATION 

88.  Study  the  reticular  formation  in  the  various  sections.  Of  what  is  it  composed? 
How  many  kinds  of  internal  arcuate  fibers  can  you  find?  What  is  the  source  of  the 
longitudinal  fibers  of  the  reticular  formation?  Locate  the  tectospinal  tract  and  in- 
dicate it  in  black  on  the  left  side  of  your  drawings.     (See  pp.  144,  145). 

89.  The  Fasciculus  Longitudinalis  Medialis. — Examine  all  nine  sections,  and  enter 
this  bundle  in  black  on  both  sides  of  your  drawings.  What  is  the  source  of  its  fibers 
and  what  is  its  function?     (See  Fig.  109;  pp.  152,  162). 

PROSENCEPHALON 

90.  With  a  sharp  brain  knife  divide  the  human  brain  exactly  in  the  median  sagittal 
plane,  and  then  cut  the  left  cerebral  hemisphere  into  a  series  of  frontal  sections.  The 
planes  of  the  sections  should  pass  through  (1)  the  rostrum  of  the  corpus  callosum, 
(2)  the  anterior  commissure,  (3)  the  mammillary  body,  (4)  the  habenular  nucleus, 
(5)  the  pineal  body  and  the  splenium  of  the  corpus  callosum  (Figs.  186-190). 

91.  Take  the  right  half  of  the  sheep's  brain  and  make  such  dissections  as  may  be 
necessary  to  secure  a  good  preparation  of  the  structures  indicated  in  Fig.  84.  Begin 
at  the  rostral  angle  of  the  fourth  ventricle  and  follow  the  cerebral  aqueduct,  tearing 
away  with  tissue  forceps  any  parts  of  the  left  lateral  wall  which  have  not  been  cut  away. 


\     I   \r.ok\lMRY    01    II.  IM      "|      mi    ,,,    xx  \|,,my 

Follow  the  aqueduct  into  the  third  ventricle,  removing  from  the  hitter  the  remai 
it-  left  lateral  wall.    Care  is  required  in  removing  the  rostral  part  of  this  wall  in 
that  the  lamina  terminalis  may  he  left  intact.     X<»w  remove  such  portions  of  the  kit 
cerebral  cortex  a- are  Mill  attached  t<>  the  preparation.     By  this  diss*  tion  a  much  mere 
instructive  preparation  is  obtained  than  when  the  original  section  is  made  exactly  in 
the  median  plane. 

{>2.  Take  the  left  lateral  hall"  of  the  sheep"-  brain  and  tear  away  what  remain-  of  the 
septum  pellucidum  and  body  of  the  fornix  and  Locate  the  «  an. late  nucleus,  lor  the 
identification  of  these  structures  see  Figs.  84  and  204.  i  Cm  through  the  internal  capsule, 
which  has  previously  been  exposed  from  the  lateral  side  in  this  specimen,  along  a  line 
extending  horizontally  toward  the  occipital  pole  from  the  highesl  part  of  the  dorsal 
border  of  the  caudate  nucleus.  Remove  the  portion  of  the  cerebral  hemisphere  that 
lies  dorsal  to  the  plane  of  this  section  and  thus  expose  the  dorsal  surface  of  the  thalamus 
(Fig.  91). 

93.  Diencephalon. — Study  the  thalamus  as  it  appears  in  all  of  these  preparation- 
(pp.  213-216).  Examine  the  dorsal  surface  of  the  thalamus  on  the  left  half  of  the  sheep's 
brain  ( Figs.  89,  91,  180).  The  lateral  surface  of  the  thalamus  rests  against  the  internal 
capsule,  as  can  be  readily  understood  from  a  study  of  this  dissection.  The  medial 
surface  forms  a  part  of  the  wall  of  the  third  ventricle  (Figs.  158,  159). 

94.  Study  the  cpithalamus  in  both  the  human  and  the  sheep's  brain.  Of  what 
parts  is  it  composed?     (See  Figs.  91,  158,  159;  pp.  220,  221.) 

95.  Locate  all  the  parts  which  belong  to  the  hypothalamus  in  both  the  human  and 
the  sheep's  brain  (Figs.  84,  86,  158,  159;  pp.  222,  223). 

96.  Study  the  shape  and  boundaries  of  the  third  ventricle  (Figs.  158,  159;  pp. 
223,  224). 

97.  The  Metathalamus. — On  the  left  half  of  the  sheep's  brain  identify  the  medial 
geniculate  body  (Fig.  87).  Immediately  rostral  to  this  body  is  a  slight  elevation  in  the 
optic  tract  produced  by  the  subjacent  lateral  geniculate  body.  Identify  both  of  these 
bodies  on  the  human  brain  (Figs.  88,  89,  154). 

98.  In  the  frontal  sections  of  the  left  human  cerebral  hemisphere  identify  the  various 
parts  of  the  diencephalon  (Figs.  188,  189).  From  these  sections  something  can  be 
learned  concerning  the  internal  st nature  of  the  thalamus,  but  more  information  can 
be  obtained  on  this  subject  from  sections  stained  by  the  Weigert  method  (Figs.  156, 
157;  p.  216).  In  these  sections  trace  the  basis  pedunculi  into  the  internal  capsule  and 
the  medial  lemniscus  into  the  thalamus. 

99.  Dissection  of  the  Optic  Trad.— Take  the  left  lateral  half  of  the  sheep's  brain 
and,  grasping  the  optic  chiasma  with  the  tissue  forceps,  pull  the  optic  tract  lateralward. 
separating  it  from  the  surface  of  the  peduncle.  It  separates  easily  until  the  position 
of  the  lateral  geniculate  body  is  reached  just  rostral  to  the  medial  geniculate  body. 
Stronger  traction  will  cause  it  to  tear  away  from  the  lateral  geniculate  body,  which  is 
now  exposed  as  a  prominent  curved  ridge  of  gray  matter.  This  nucleus  extends  r.-strally 
and  dorsally  from  the  medial  geniculate  body  and  is  continuous  with  the  pulvinar  of 
the  thalamus.  Continued  traction  will  cause  the  optic  fibers  to  strip  off  from  the  sur- 
face of  the  pulvinar.  Here  they  form  a  rather  thick  white  lamina,  the  stratum  ztinale. 
Continue  the  dissection,  raising  the  fibers  of  the  optic  tract  as  far  as  the  groove  rostral  to 
the  superior  colliculus.     Now  cut  the  transverse  peduncular  tract,  which  lies  in   this 


37° 


IIII.    NERVOUS    SYSTEM 


groove,  by  making  a  superficial  incision  across  the  groove  along  the  lateral  border  of 
the  optic  fibers.  Scrape  away  the  superficial  gray  matter  (about  1  mm. )  of  the  superior 
colliculus  and  expose  the  stratum  opticum  (Fig.  116).  Now  continue  the  traction  on 
the  optic  tract  and  a  striking  demonstration  will  be  obtained  of  the  fact  that  the  stratum 
opticum  is  composed  of  fibers  from  this  tract  (Figs.  161,  162;  pp.  226,  227). 

100.  Dissection  of  the  Optic  Radiation. — In  the  left  half  of  the  sheep's  brain  scrape 
away  part  of  the  ,Lrray  matter  of  the  pulvinar.  Follow  fibers  from  the  pulvinar  into  the 
posterior  limb  of  the  internal  capsule.  These  belong  to  the  optic  radiation,  which  may 
now  be  followed  by  dissection  to  the  cortex  near  the  occipital  pole  of  the  cerebral  hemi- 
sphere (Fig.  260;  pp.  227,  228).  Now  take  the  right  half  of  the  cerebral  hemisphere 
and  identity  the  visual  area  of  the  cerebral  cortex  (Fig.  221;. 


Optic  radiation  ' ' 
Superior  colliculus-'  » 
Inferior  colliculus  '  ■ 
Pulvinar '  [ 
Medial  geniculate  body 
Cerebral  peduncle 


Mam  miliary  body   ! 

Optic  tract    > 

Posterior  limb  of  internal  capsule        • 

Optic  nerve 


;    ;      Intersection  of  corona  radiata  and 
'<    !  radiation  of  corpus  callosum 

\   'Anterior  limb  of  internal  capsule 
Anterior  perforated  substance 


Fig.  260. — Dissection  of  the  cerebrum  of  a  sheep  showing  the  internal  capsule  and  corona  radiata. 
The  lentiform  nucleus  has  been  removed. 


101.  Surface  Form  of  the  Cerebral  Hemispheres. — Compare  the  basal  surface  of  the 
human  brain  with  that  of  the  sheep.  Note  in  each  the  parts  belonging  to  the  rhinen- 
cephalon  and  locate  the  rhinal  fissure,  which  separates  the  neopallium  and  the  archi- 
pallium.  Nearly  all  of  the  surface  of  the  human  cerebral  hemisphere  is  formed  by  the 
neopallium  (Figs.  83,  86;  pp.  115,  116). 

102.  Examine  the  right  cerebral  hemisphere  of  the  human  brain  and  identify  the 
poles,  fissures,  sulci,  lobes,  and  gyri  (Figs.  166-168,  170,  171;  pp.  232-242).  Draw 
the  margins  of  the  lateral  fissure  apart  and  locate  the  insula  (Fig.  169).  Study 
the  insula  in  the  frontal  sections  through  the  left  cerebral  hemisphere  (Figs.  186-189; 
p.  237). 

103.  Internal  Configuration  of  the  Cerebral  Hemisphere. — Take  the  sheep's  brain 
from  which  the  cerebellum  has  been  removed  and  slice  away  successive  thin  layers  from 
the  dorsal  aspect  of  both  hemispheres.  These  thin  sections  should  be  cut  in  planes 
parallel  to  the  dorsal  surface  of  the  corpus  callosum  and  the  last  cut  should  be  }  inch 
dorsal  to  that  commissure.     The  direction  and  relative  depth  of  the  dorsal  surface  of 


\    LABOB  \K)KV    in   I  i.i\i.    01     \i  i  RO   \\  \Im\in  71 

the  corpus  callosum  can  be  determied  by  examination  of  the  medial  aspecl  of  the  right 
half  of  the  sheep's  brain.  As  the  sections  are  removed  note  the  relation  of  the  gray 
andwhite  matter  I  Fig.  175).  Gently  pressaparl  the  two  hemisphen  and  note  the<  orpus 
callosum  at  the  bottom  of  the  longitudinal  fissure.  Now  with  a  blunl  instrument 
dissect  away  the  gray  and  white  matter  from  the  dorsal  surfai  e  of  the  1  orpus  1  alio  um 
(Fig.  175).  Be  careful  not  to  injure  a  thin  layer  of  gray  matter,  the  indusium  griseum, 
which  covers  this  surface.  Study  the  corpus  callosum  in  this  specimen  and  in  the  median 
sagittal  sections  of  the  sheep  and  human  brains  (Figs.  158,  159,  175;  pp.  243  245). 
Examine  the  septum  peUucidum  in  the  median  sagittal  sections. 

104.  The  Lateral  Ventricles  (pp.  246  251). — Cut  through  the  corpus  <  allosum  of  the 
sheep's  brain  as  indicated  in  Fig.  ITS,  leaving  a  median  strip  in  position.  Male  a 
careful  examination  of  all  the  parts  thus  exposed,  including  the  septum  pellucidum. 
On  the  right  side  of  the  specimen  expose  the  entire  extent  of  the  inferior  horn  of  the 
lateral  ventricle  by  freely  cutting  away  the  lateral  portion  of  the  hemisphere  a-  indi<  ated 
in  Fig.  182.  Remove  the  caudate  nucleus  to  demonstrate  the  entire  extent  of  the  ante- 
rior horn,  and  finally  demonstrate  the  continuity  of  the  lateral  ventricle  with  the  cavity 
of  the  olfactory  bulb  (Fig.  182).  Now  study  the  lateral  ventricle  and  the  structures 
which  form  its  walls  as  these  are  illustrated  on  the  twro  sides  of  this  specimen.  Note 
the  chorioid  plexus  (Fig.  183)  and  chorioid  fissure. 

105.  Study  the  lateral  ventricle  as  seen  in  the  frontal  sections  of  the  left  hemi- 
sphere of  the  human  brain  (Figs.  186-189).  It  has  an  additional  part,  the  posterior 
horn,  not  seen  in  the  sheep.  Endeavor  to  reconstruct  a  mental  picture  of  its  shape 
(Fig.  176). 

106.  The  Corpus  Striatum  (pp.  253-257). — Examine  again  the  caudate  nucleus  as 
it  bulges  into  the  lateral  ventricle  (Fig.  178).  Take  the  right  lateral  half  of  the  sheep's 
brain  and  make  a  horizontal  section  through  the  cerebral  hemisphere,  passing  through 
the  lower  border  of  the  genu  of  the  corpus  callosum  and  the  lower  border  of  the  habenular 
trigone.  Locate  the  lentiform  and  caudate  nuclei,  the  claustrum,  and  the  internal 
and  external  capsules  (Fig.  192). 

107.  Dissection  of  the  Lentiform  Nucleus  and  the  Internal  Capsule.— On  the  left 
side  of  the  sheep's  brain,  in  which  the  lateral  ventricles  have  been  exposed,  remove  the 
cortex  and  white  matter  superficial  to  the  lentiform  nucleus.  Begin  by  grasping  with 
tissue  forceps  the  olfactory  bulb  close  to  its  peduncle  and  tear  it  away,  pulling  in  a 
lateral  and  caudal  direction.  There  should  come  away  with  it  the  superficial  part  of 
the  anterior  perforated  substance  and  part  of  the  lateral  olfactory  gyrus  (Fig.  83). 
This  will  expose  the  ventral  part  of  the  lentiform  nucleus,  and  the  structures  lateral 
to  that  nucleus  can  now  be  removed.  With  a  blunt  dissecting  instrument  scrape  away 
everything  superficial  to  the  lentiform  nucleus  and  continue  the  dissection  until  the 
nucleus  and  the  corona  radiata  are  fully  exposed  (Fig.  87).  Now  scrape  away  the 
lentiform  nucleus  and  expose  the  internal  capsule  (Fig.  260).  In  removing  the  nucleus 
you  can  obtain  a  clear  idea  of  its  shape  and  size. 

108.  Dissection  of  the  Internal  Capsule.— In  the  same  specimen  remove  the  optic 
tract  and  trace  the  basis  pedunculi  into  the  internal  capsule  and  follow  the  libers  from 
the  internal  capsule  into  the  corona  radiata.  Trace  the  optic  radiation  from  the  poste- 
rior extremity  of  the  internal  capsule  to  the  cortex  near  the  occipital  pole  (Fig.  260). 

109.  Dissection  of  the  Caudate  Xueleus.— On  the  left   side  of  the  same  sheep's 


372  THE    NERVOUS    SYSTEM 

brain  note  that  the  tail  of  the  caudate  nucleus  extends  ventrally  into  the  roof  of  the 
inferior  horn  of  the  lateral  ventricle.  With  a  blunt  instrument  scrape  away  the  head 
and  first  part  of  the  tail  of  the  nucleus,  exposing  the  medial  surface  of  the  internal  cap- 
sule (Fig.  91).     Note  the  shape  and  size  of  this  nucleus  as  you  are  removing  it. 

110.  Study  a  horizontal  section  stained  by  the  Weigert  method  through  the  internal 
capsule  and  basal  ganglia.  From  this  section  and  from  the  dissections  endeavor  to 
form  a  clear  mental  picture  of  the  internal  capsule  and  its  relations  (Figs.  191,  193; 
pp.  257-261). 

111.  Now  take  the  frontal  sections  of  the  left  hemisphere  of  the  human  brain 
and  identify  the  various  parts  of  the  corpus  striatum  and  internal  capsule  (Figs.  186- 
190). 

112.  Rhinencephalon. — Study  the  olfactory  portions  of  the  brain  to  be  seen  on  the 
ventral  surface  of  the  cerebral  hemisphere  in  the  human  and  sheep's  brains  (Figs.  172, 
197, 199;  pp.  265-269).  Study  the  hippocampus,  alveus,  and  fimbria  as  they  lie  exposed 
in  the  inferior  horn  of  the  lateral  ventricle  of  the  sheep's  brain  (Figs.  178,  182).  Open 
up  the  inferior  horn  of  the  lateral  ventricle  on  the  left  side  of  this  specimen  so  as  to 
expose  the  hippocampus  and  fimbria.  Raise  the  hippocampus  and  fimbria  on  both  sides 
at  the  same  time,  leaving  them  still  attached  to  the  fornix.  This  should  be  done  without 
damaging  the  underlying  tela  chorioidea  of  the  third  ventricle,  which  occupies  the  great 
transverse  fissure.  Examine  the  under  surface  of  the  hippocampus,  fimbria,  and  for- 
nix. Note  that  the  two  fimbriae  unite  to  form  the  triangular  body  of  the  fornix. 
The  transverse  fibers  in  this  triangle  constitute  the  hippocampal  commissure  (lyra). 
Note  the  fascia  dentata  and  hippocampal  fissure.  Figure  204  will  help  you  to  interpret 
the  parts  seen  in  this  dissection. 

113.  The  chorioid  plexuses  of  the  prosencephalon  are  now  fully  exposed,  and  their 
relations  to  each  other  and  the  brain  ventricles  can  be  readily  studied  (pp.  224,  251). 

114.  Remove  the  tela  chorioidea  of  the  third  ventricle  and  again  identify  the  parts 
of  the  thalamus  and  epithalamus  which  may  be  seen  from  above  (Figs.  91,  180). 

115.  Replace  the  fornix  and  hippocampus  in  position  and  divide  the  fornix  and  what 
remains  of  the  cerebral  hemispheres  by  a  sagittal  section  i  millimeter  to  the  right  of 
the  median  plane.  Take  the  left  half  of  the  preparation  and,  tearing  away  any  por- 
tions of  the  right  columna  fornicis  that  may  still  be  attached  to  the  preparation,  follow 
the  left  column  of  the  fornix  to  the  mammillary  body.  This  can  be  accomplished  by 
scraping  away  some  of  the  medial  surface  of  the  thalamus  (Fig.  204).  At  the  same  time 
expose  the  mamillothalamic  tract.  Remove  the  posterior  part  of  the  thalamus  and  the 
remainder  of  the  brain  stem  by  a  cut  made  just  caudal  to  the  mamillothalamic  tract, 
as  indicated  in  Fig.  204.  This  gives  a  connected  view  of  the  entire  fornix  system. 
Find  the  cut  surface  of  the  hippocampal  commissure  and  separate  it  for  a  few  milli- 
meters from  the  rest  of  the  fornix.  Identify  again  the  fimbria,  fascia  dentata,  hippo- 
campal fissure  and  hippocampal  gyrus,  and  study  the  fornix  as  a  whole  (Figs.  200, 
203;  pp.  270-272). 

116.  Study  the  septum  pellueiduni  in  the  right  half  of  the  human  brain  (Fig.  158; 
p.  272).     Also  locate  the  anterior  commissure. 

117.  Dissect  the  anterior  commissure  in  the  right  lateral  half  of  the  sheep's  brain. 
Locate  the  commissure  on  the  median  surface  and  by  blunt  dissection  follow  it  to  the 
olfactory  bulb  (Fig.  199;  p.  273). 


A     LABOK  \K>K'V    Ml    I  |.!\i:    OF     M   I    I  m    \\  \|,,\iv 

118.  In  the  frontal  sections  of  the  left  cerebral  hemisphere  of  the  human  I. rain  study 
the  relations  of  the  septum  pellucidum,  fornix,  fimbria,  hippocampus,  and  anterior 
commissure  (Figs.  186  190). 

11<).  The  Cerebral  Cortex.    On  the  right  hemisphere  of  the  human  I. rain  identify 
the  motor,  somesthetic,  auditory,  and  visual  centei      I  igs.  JJO,  221;  pp.  290 
With  a  scalpel  remove  a  cube  of  cortex  and  subjacent  white  matter  from  ea<  h  of  these 

areas.      Each    Mock   should   measure  about    1    em.    in   each   dimension.      Willi   a    sharp 

razor  make  section  through  each  oi  these  blocks  a1  righl  angles  to  the  surfa<  e  of  thee 
and  perpendicular  to  the  long  axis  of  the  gyrus  from  which  the  block  wascut.  Not  the 
differences  in  thickness  of  the  cortex  in  the  various  regions.  Observe  the  white 
striatums  in  the  cortex,  and  note  how  these  differ  in  the  several  specimens  (Fig. 
Study  the  stained  and  mounted  sections  of  the  cerebral  cortex  which  are  furnished 
you.  What  details  of  cell  and  fiber  lamination  do  these  preparation-  show,  and  how- 
does  this  lamination  differ  in  the  several  regions  of  the  cortex?  (See  Fig.  215;  pp. 
284-287.) 

120.  Association  Fibers  (Figs.  226,  228;  pp.  298-301).— If  the  human  brain  is  reason- 
ably well  preserved  the  larger  bundles  of  association  fibers  may  be  easily  exposed  by 
dissection.  This  can  be  done  on  the  right  hemisphere.  But  if  the  material  is  very 
soft  this  half  of  the  brain  can  more  profitably  be  laid  into  a  series  of  horizontal  sections 
and  these  used  for  a  review  of  the  form  and  relations  of  the  component  parts  of  the 
cerebral  hemisphere.  If  the  material  is  fairly  well  preserved,  make  the  following 
review  dissection  and  at  the  same  time  expose  and  study  the  various  bundles  of  asso- 
ciation fibers. 

121.  Review  Dissection  of  the  Human  Brain. — Take  the  right  half  of  the 
human  brain  and  scrape  away  the  cerebral  cortex  from  a  portion  of  the  dorsal 
surface  of  the  frontal  lobe.  This  will  expose  the  short  association  or  arcuate  fibers 
(Fig.  226). 

122.  Now  make  a  horizontal  section  through  the  hemisphere  parallel  to  the  dorsal 
surface  of  the  corpus  callosum  and  f  inch  dorsal  to  it.  Note  the  centrum  semiovale. 
Scrape  away  the  cortex  of  the  gyrus  cinguli  and  the  white  matter  immediately  sub- 
jacent to  it.  In  making  this  dissection  carry  the  orangewood  stick  in  an  anteroposterior 
direction,  removing  the  white  matter  a  little  at  a  time  until  a  longitudinal  bundle  of 
fibers,  the  cingulum,  is  exposed  (Fig.  174).  The  indusium  griseum  and  stria'  longi- 
tudinales  should  now  be  uncovered. 

123.  Remove  the  cingulum,  scrape  away  the  indusium  griseum,  and  expose  the 
radiation  of  the  corpus  callosum  as  indicated  on  the  right  side  of  Fig.  174,  but  do  not 
cut  the  optic  radiation  or  expose  the  tapetum  at  this  time. 

124.  Remove  the  parietal  operculum  a  little  at  a  time.  This  can  be  done  with 
tissue  forceps.  Grasp  small  portions  and  tear  them  away  by  upward  traction.  Note 
the  bundles  of  transverse  fibers  which  enter  this  operculum  from  the  corpus  callosum 
and  internal  capsule.  These  intersect  at  right  angles  with  the  libers  of  the  superior 
longitudinal  fasciculus  which  should  come  into  view  as  the  dissection  progresses 
174).  The  transverse  bundles  should  be  made  to  break  off  at  the  point  where  they  pass 
through  the  superior  longitudinal  fasciculus.  Complete  the  dissection  of  this  fasciculus, 
carrying  the  dissecting  instrument  in  the  direction  of  its  fibers.  Now  demonstrate  the 
intersection  of  the  corona  radiata  with  the  radiation  of  the  corpus  callosum  (Fig.  174). 


374  THE    NERVOUS    SYSTEM 

By  this  dissection  the  insula  and  the  dorsal  surface  of  the  temporal  lobe  have  been  ex- 
posed.   Note  in  particular  the  transverse  temporal  gyri. 

125.  Now  dissect  away  the  dorsal  part  of  the  temporal  lobe  and  remove  the  insula. 
This  will  expose  the  uncinate  and  inferior  occipitofrontal  fasciculi  as  well  as  the  external 
capsule  (Fig.  227).  These  fiber  bundles  can  best  be  displayed  by  carrying  the  dis- 
secting instrument  in  the  direction  of  the  fibers.  Complete  the  dissection  of  the  corona 
radiata  and  the  optic  radiation  (Fig.  227). 

126.  Now  turn  the  specimen  over  and  make  a  dissection  of  the  column  of  the  fornix 
and  the  mamillothalamic  tract  as  in  Fig.  205,  but  do  not  cut  away  the  brain  stem  as 
indicated  in  that  figure. 

127.  Dissection  of  the  Internal  Capsule  from  the  Medial  Side  (Fig.  195). — Tear 
away  the  fornix  and  septum  pellucidum,  opening  up  the  lateral  ventricle.  With  the 
brain  knife  cut  away  a  slice  from  the  medial  surface  of  the  hemisphere,  varying  in  thick- 
ness from  t  inch  at  the  frontal  end  to  \  inch  at  the  occipital  end,  cutting  through  the 
corpus  callosum  and  into  the  ventricle,  but  not  into  the  basal  ganglia.  With  a  scalpel 
and  tissue  forceps  remove  what  remains  of  the  medial  wall  of  the  lateral  ventricle, 
except  in  the  inferior  horn.  Grasp  with  tissue  forceps  the  stria  terminalis  in  the  rostral 
end  of  the  sulcus  terminalis  and  tear  it  away,  carrying  the  forceps  toward  the  occipital 
pole  (p.  214).  By  blunt  dissection  remove  the  thalamus  and  subthalamus  as  well  as  the 
tegmentum  and  corpora  quadrigemina  of  the  mesencephalon.  In  scraping  away  these 
parts  carry  the  dissecting  instrument  from  the  sulcus  terminalis  in  a  ventral  direction. 
This  will  uncover  the  basis  pedunculi  and  its  continuation  into  the  internal  capsule. 
The  fibers  of  the  thalamic  radiation  will  be  broken  off  at  the  point  where  they  enter  the 
internal  capsule  (Fig.  195).  Remove  the  ependymal  lining  of  the  posterior  horn  of  the 
ventricle  and  uncover  the  tapetum.  Scrape  away  the  caudate  nucleus,  carrying  the 
dissecting  instrument  in  the  direction  of  the  fibers  of  the  internal  capsule  (Fig.  195). 
Trace  the  anterior  commissure  to  the  point  where  it  disappears  under  the  anterior 
limb  of  the  internal  capsule.  Study  the  internal  capsule  as  seen  from  the  medial  sur- 
face, and  note  particularly  the  direction  of  the  fibers,  the  anterior  limb,  the  posterior 
limb,  the  optic  radiation,  and  the  curved  ridge  which  represents  the  genu* 

128.  Now  turn  again  to  the  lateral  side  of  the  specimen  (Fig.  227),  and  grasping 
with  tissue  forceps  individual  strands  of  the  uncinate  fasciculus  in  temporal  lobe  strip 
them  forward  into  the  frontal  lobe.  Remove  the  entire  fasciculus  in  this  manner.  In 
the  same  way  strip  away  the  fibers  of  the  inferior  occipitofrontal  fasciculus,  beginning 
in  the  frontal  lobe  and  tracing  them  toward  the  occiput.  Strip  off  the  fibers  of  the  ex- 
ternal capsule  and  expose  the  lentiform  nucleus  and  the  corona  radiata  (Fig.  194). 
Pay  special  attention  to  the  fibers  of  the  corona  radiata  which  come  from  the  sublen- 
ticular part  of  the  internal  capsule  and  enter  the  temporal  lobe.  Follow  the  anterior 
commissure  to  the  point  where  it  disappears  under  the  lentiform  nucleus. 

129.  Remove  what  remains  of  the  temporal  lobe  and  examine  the  hippocampus, 
fimbria,  and  inferior  horn  of  the  lateral  ventricle  from  the  dorsal  surface  (Fig.  201). 

130.  Next  scrape  away  the  lentiform  nucleus  and  trace  the  basis  pedunculi  into  the 
internal  capsule  (Fig.  88).  Study  the  corona  radiata,  internal  capsule,  and  basis 
pedunculi  from  both  sides  of  this  preparation.  The  thalamus  and  the  caudate  and 
lentiform  nuclei  produce  well-marked  impressions  on  the  internal  capsule  (Figs.  88,  195). 


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INDEX 


Note.  -In  cross  references  the  key  words  arc  italicized. 
the  pages  on  which  the  structures  arc  illustrated. 


The  numbers  in  Italics  refer  t<> 


Accommi  idation  "t  \  ision,  5  $2 

Acoustic  ana  of  cortex.     See  Center,  auditory 

Acousticolateral  area,  358 

Affenspalte,  237 

Ala  cinerea,  127 

lobuli  centralis,  197 
Alveus,  27(i,  27s,  279 
Ameba,  17 

Amnion's  horn.     Sec  Hippocampus. 
Ampulla  of  semicircular  canal,  358 

Amygdala.     See  Nucleus,  amygdaloid. 
Ansa  lent  icularis,  2<>S 

peduncularis,  2(>,:! 
Aperture,  lateral,  of  fourth  ventricle,  125 

medial,  of  fourth  ventricle,  125 
Apex  col u mnae  posterioris,  7(» 
Aphasia,  295 
Aqueductus  cerebri  (aqueduct  of  Sylvius),  26, 

158 
Arachnoid,  73 
Arbor  vitas,  199 

Archipallium,  116,  242.  270,  277,  278,  279 
Area,  acousticolateral,  358 
acustica,  127 

cortical,  287.     (See  also  Center.) 
oval,  of  Flechsig,  107 
parolfactoria  of  Broca,  267 
postrema,  129 
pyriform,  116,  268,  277 
striata,  293 
Association  bundles  of  cerebrum,  298 
arcuate,  298,  300 
cingulum,  299 
inferior  longitudinal,  299 

occipitofrontal,  300 
superior  longitudinal,  300 

occipitofrontal,  301 
uncinate,  299 
Ataxia,  99 

Auditor\  apparatus,  186,  309 
Auerbach's  plexus,  351 
Autonomic  system,  339 
cranial,  339 
craniosacral,  340,  354 
sacral,  339 

thoracolumbar,  339,  354 
Axon  (axis-cylinder),  37,  43,  45 

hillock.     See  Cone,  implantation. 
Axonal  reaction.     See  Chromatolysis. 

BAILLARGER,  lines  of,  283 
Band,  diagonal,  267 
Basis  cerebri,  115,  1 20 

pedunculi,  129,  158,  164 
Basket-cells,  209 
Bell's  law,  60 


Betz,  cells  of,  290 
Bladder,  innervation  of,  $54 
Body  of  cell,  43 
oi  fornix,  27 1 

geniculate,  lateral,    131,  220 
medial,    1  $1,    107,   220 

mammillary,  111.  280 

of  Nissl,  48,  51 

paraterminal,  267 

pineal,  221 

pituitary.     See  Hypophysis. 

quadrigeminal,  130,  165 

restiform,  122,  143,  205 

striate.     See  Corpus  striatum. 

tigroid.     See  Nissl  body. 

trapezoid,  121,  150,  186 
Brachium  (or  brachia),  conjunctivum,  125,  155, 
159,  160,  206,  211 

of  corpora  quadrigemina,  131 

pontis,  123,  204 

quadrigeminum  inferius,  131,  163,  166 
superius,  131,  167 
Brain,  56,  115 

development,  25 

divisions  of,  2F< 

end-.     See  Telencephalon. 

fore-.     See  Prosencephalon. 

hind-.     See  Metencephalon  and  Rhombenceph- 
alon. 

inter-.     See  Diencephalon. 

stem.     See  Medulla  oblongata,  Pons,  Mesen- 
cephalon, and  Ganglia,  basal. 

vesicles,  24,  25 

weight,  301 
Broca's  convolution,  235 
Brown-Sequard  syndrome,  112 
Bulb,  olfactory,  265,  274 

of  posterior  horn,  248 
Bundle.     (See  also  Fasciculus  and  Tract.) 

association,  of  cerebrum,  298,  299,  500 

cornucommissural,  107 

ground.     See  Fasciculus  proprius. 

ol  Gudden,  tegmental.     See  Tract,  mammillo- 
tegmental. 

marginal.     See  Fasciculus  dorsolateralis. 

Oval.      See  .1  red.  oval. 

posterior  longitudinal.    See  Fasciculus,  medial 
gil  udinal. 

of  Turck.     See  Tract,  ventral  corticospinal. 

ventral  longitudinal.     See  Tract,  tectospinal. 
Burdach,  column  of.      See  Fasciculus  cuneatus. 

nucleus  of.     See  Nucleus  cuneatus. 

CAJAL,  commissural  nucleus  of,  330 

horizontal  cells  of,  285 
Calamus  scriptorius,  127 

383 


3»4 


INDEX 


Calcar  avi- 

Canal,  central  fcanalis  centralis),  80,  136 
lateral  line,  -  - 
semicircular,  315,  356 
spinal,  73 
Capsule,  external,  2  :  7 
internal,  257,  259,  261 
nasal 

of  spinal  ganglion  eel!,  63 
Cauda  equina,  78 
Cavum  septi  pellucidi,  272 
Cell.     (See  also  Neuron.) 
basket,  209 
of  Betz,  290 
body,  43 

ependymal,  37,  85 
germinal.   $7 
granule,  of  cerebellum,  208 

of  cerebral  cortex.     See  Neurons,  stellate. 

of  olfactory  bulb,  276 
mitral,  27 
neuroglia,  85,  86 
of  Purkinje,  207 
pyramidal,  285 
Cell-columns  of  Clarke.    See  Nucleus  dorsalis. 
intermediolateral,  89 
of  spinal  cord,  89,  go 
Center,  cortical,  290 

association,  293 

auditory,  293 

motor,  290,  317,  318 

olfactory,  293 

opti. 

projection,  290 

somesthetic,  292 

of  speech,  295 

visual,  292 
for  pain,  219 
projection.  290 
respiratory,  330 
Central  nervous  system,  20,  21,  56,  57 
Centrum  medianum  thalami,  218 

semiovale,  243 
Cerebellum,  195 

in  birds  and  reptile- 
central  white  matter,  199 
cortex,  199,  206,  207,  208,  209 
development  of,  195 
in  the  dogfish,  27,  28 

fiber  tracts  of,  204.  205,  206,  209,  210,  211 
folia,  199 

hemispheres  of,  197,  198 
histogenesis,  196 
laminae,  199 

lobes  or  lobules,  197,  198,  200,  201,  202 
in  mammals,  200 
microscopic  structure,  206 
morphology  of,  199 
notches,  197 
•  nucleus  dentatus,  203,  211 

emboliformis,  2 

fastigii  or  tecti,  204,  211 

globosus,  20 } 
peduncles,  204 

inferior,  122,  143,  205 

middle,  123,  204 

superior,  125,  155,  159,  160,  206,  211 
section,  median,  199 

through  hemisphere,  199 


Cerebellum  in  the  sheep,  200,  201,  202 
vermis  of,  196 
white  matter,  199 
Cerebral  aqueduct.     See  Aqueduttus  cerebri, 
cortex,  114,  232,  283 
area  of,  acoustic,  293 
association,  293 
audito-psychic,  293,  294 
audito-sensory,  294 
of  Broca,  295 
motor,  290,  317,  318 
striata,  293 

visuo-psychic,  293,  294 
visuo-sensory,  294 
centers  of,  290,  292,  2<>? 
development,  2  50 
electric  excitability  of,  291 
frontal  olfactory,  277 
hippocampal,  278,  279 
histogenesis,  230 
layers  of,  286,  287 
localization  of  function  in,  290 
myelination  of  fibers,  289 
nerve-cells,  284,  285 
nerve-fibers,  283,  284 
neuroglia-cells,  284 
structure,  283,  284,  285,  286 
hemispheres,  113,  229,  232 
borders,  232 
commissural  fibers,  296 
convolutions.     See  Gyri. 
corticifugal  or  efferent  fibers,  283 
corticipetal  or  afferent  fibers,  283 
development,  25,  32,  22') 
in  the  dogfish,  27,  28,  30 
external  conformation,  229 
fissures.     See  Fissure. 
gyri.     See  Gyrus. 
lobes.     See  Lobe. 
lobules.     See  Lobule. 
medullars"  center,  243,  296 
pallium,  25,  32,  33,  229 
poles,  232 
sulci.     See  Sulcus. 
surfaces,  232 
ventricles,  lateral,  246 
peduncles.     See  Peduncles. 
vesicles,  24,  25 
Cerebrospinal  fluid,  73,  126 

system.  r : 
Cerebrum,  117 

Cervix,  columnar  posteriori-.  79 
Chiasma,  optic,  223,  220 
Chorda  tympani,  192,  3r2 
Chorioid  fissure,  229,  251 
plexuses.     See  Plexus. 
Chromatolysis,  51 

Chromophilic  bodies.     See  Nissl  bodies. 
Cingulum,  299 

Clarke,  column  of.     See  Nucleus  dorsalis. 
Claustrum,  256 
Cava,  121,  137 
Climbing  fibers,  209,  210 
Clivus  monticuli.     See  Declke  monticuli. 
Cochlea,  185 

nterates,  19 
Cold,  sensations  of,  105,  306 
Collateral  fiber-.  43,  ')7 
|  Colliculus  facialis,  127 


INDI   \ 


Colliculus,  inferior,  130,  165 

superior,  130,  165,  U>7 
Column,  anterior,  80 

ol  Burdach.     See  Fasciculus  cuneatus. 

nt  Clarke.     See  Nucleus  dorsalis. 

dorsal  (columna  <  1< u >-.i  1  i^  gi  isea  I,  42 

of  fornix,  272 

ol'  (.oil.     See  Fasciculus  gracilis. 

pay,  76 

intermediolateral,  89 

lateral,  80 

nuclear,  of  brain  stem,  168,  170,  171,  174 

posterior,  79 

somatic  afferent,  170,  182,  185 
efferent,  17o 

ventral,  42,  80 

vesicular.     See  Nucleus  dorsalis. 

visceral  afferent,  170,  180 
efferent,  170,  174,  177 
Comma  tract  of  Schultze.    See  Fasciculus  inter- 

fascicularis. 
Commissura  anterior  alba,  80 

habenularum,  220 

Commissure  or  commissures,  anterior  cerebri, 
22\  231,  273,  296 
gray,  80 
white,  80 

great  transverse.     See  Corpus  callosum. 

of  Gudden,  227 

habenular,  22(i 

hippocampal,  231,  271,  280,  296 

of  inferior  colliculi,  159 

middle.     See  Afassa  intermedia. 

optic.     See  Chiasma,  optic. 

posterior,  of  cerebrum,  221 
of  spinal  cord,  80 

superior.     See  Commissure,  habenular. 
Components    of    nerves,    61,     168.     (See    also 

Nerve-fibers. ) 
Conduction  of  nerve  impulses,  50 
Cone,  implantation,  44 

of  origin.     See  Cone,  implantation. 
Cones  of  retina,  226 
Consciousness,  2^,  302 
Conus  medullaris,  74 
Convolution.     See  Gyrus. 
Coordination,  99,  210,  311 
Cornu  ammonis.     See  Hippocampus. 
Cornucommissural  bundle,  107 
Corona  radiata,  261 
Corpus  (or  corpora)  callosum,  243,  296 
development,  231 

fornicis,  271 

geniculatum  laterale,  220 
mediate,  131,  167,  220 

mamillaria,  222,  230 

pineale,  221 

ponto-bulbare,  123 

quadrigemina,  130,  165 

restiforme,  122,  143,  205 

striatum,  25.  32,  33,  256,  262,  324 

subthalamicum  (Luysi),  22^-~ 

trapezoideum,  121,  150,  186 
Cortex,  cerebellar,  199,  206.  207.  208,  209 
localization  of  function  in,  202 
neurons  of,  207,  208.  209 

cerebral.     See  Cerebral  cortex. 
Corti,  ganglion  of.     See  Ganglion,  spiral. 

25 


<  oi  i  i,  org  in  of, 

( 'ough,  me<  nanism  of,  <^1 

'  i  rebri.   See  Peduncle,  <  erebral. 

lor  nil  i-,  27  1 

Crusta.     S  •  dunculi. 

Culmen  mom  iculi,  ' 

Cuneate  tuben  le,  121,  1  <7 

Cuneus,  239 

Cup,  opt  ic,  32,  3  J,  22r< 

(  v  toplasm  ol  nerven  ells,  42,  1 . 

I  >i  i  i  i\i   monticuli,  ' 

<ti"ii    dei  ussatio)  of  brachium  conjunc- 
tivum,  156,  l ;'' 

dorsal  t(  gmental,  161,  167 

ot  fillet.     See  Decus  ation  of  lemnu 

ot  I  orel.     See  Decussation,  ventral  tegmental. 

fountain.     See  Decussation,  dorsal  tegmental. 

of  lemniscus  (lemniscorum),  154,  138 

of  .\ie>  nert .  See  Da  ussation,  dorsal  tegmental. 

optic.     See  Chiasma,  optic. 

of  pyramids,  11''.  120,  154.  156 

tegmental.       See   Decussations,  ventral  and 
dorsal  tegmental. 

ventral  tegmental,  161 
Degeneration  of  fiber  tracts,  105,  106,  107 

of  nervc-nlx-r>,  51,  52 

Wallerian,  105,  106,  107 
Deiters,  nucleus  of,  151,  189 
Dendrites  or  dendrons,  43 
Dermatome,  58 

I  >i  \  elopment  of  the  nervous  svstem,  24,  31 
Diencephalon,  24,  2'-.  26.  28,  31,  33,  213 
Digitationes  hippocampi,  269 
Dogfish,  brain  of,  26,  27,  2S 
Dogiel's  Type  II  cells,  65 
Dura  mater.  7  J 
Dynamic  polarity,  law  of,  50 

Earthworm,  nervous  system  of,  19 

Edinger-Westphal  nucleus,  178 

Effector,  18,  19,  54,  91 

Embrvologv  of  nervous  svstem,  31,  37,  195,  215. 

22')' 
Eminentia  cinerea.     See  Ala  cinerea. 

collaterals,  250 

facialis.     See  Colliculus  facialis. 

hypoglossi.     See  Trigonum  hypoglossi. 

medialis,  129 

teres.     See  Eminentia  medialis. 
Encephalon.     See  Brain. 
End-brain.     See  Telencephalon. 
End-plates,  motor,  62 
Ependyma.  8 ; 
Epiphysis,  29,  51 
Epithalamus,  29,  55,  220 
Exteroceptor,  exteroceptive,  66,  182,  185 
l>.  •  .  development,  22> 

innervation,  225 

retina,  225 

1"  \-<  i  \    dentata,  269,  276 

Fasciculus,  65.       S<-c  also  Tract  and  Bundle.) 
anterior  proprius,  1<>7 
anterolaterals  superficialis,  100 
arcuatus,  300 

cerelxllospinalis.    See  Tract,  dorsal  spinocere- 
bellar, 
cerebrospinalis.     See  Tract,  corticospinal. 


?86 


IM'I  X 


ulus  cerebrospinal,  anterior.     Sec  Tract, 
\  entral  corticospinal. 

lateralis.    See  Trad,  lateral  corticospinal, 
cuneat  us,  7o,  8  \  95,  96,  121,  137 
dorsal  longitudinal    Schutz),  2\<> 
dorsolati  ri  -,  104 

ilis,  76,  83,  96,  121,  137 
interfascicularis,  ''7,  Ki7 
lateralis,  minor,  1  21 

proprius,  107 
longitudinalis  inferior,  2'>'> 

medialis,  145,  152,  162,  190,  328 

superior,  300 
medial  longitudinal,  145,  152,  102,  190,  328 
of  Meynert,  220 
occipitofrontalis,  inferior,  300 

superior,  301 
peduncularis  transA 
posterior  longitudinal.    See  Fasciculus,  medial 

longitudinal. 
proprius  of  spinal  cord,  107 
pyramidal.     See  Tract,  corticospinal. 
retroflexus,  220 
septomarginal,  97,  107 
solitarius  152,  181,  330 
sulcomargihalis,  ll|v 
superior  longitudinal,  300 
thalamomamillaris.       See   Tract,   mammillo- 

thalamic. 
uncinatus,  299 
Fibers,  fibrae.     (See  also  Xerce-fibers.) 
arcuate,  of  cerebrum,  299 

of  medulla  oblongata,  139 

external,  121,  123,  139,  140,  143 
internal,  154,  138,  139 
association,  92,  298 

cerebello-olivarv.    See  Fibers,  olivocerebellar, 
climbing,  209,  210 
commissural,  296 
mossy,  209,  210 

olivocerebellar,  139,  142,  143,  205 
pontis,  147 

postganglionic,  537,  343 
preganglionic,  55  7,  544 
projection,  297 

propria?.     See  Fibers,  arcuate,  of  cerebrum, 
recta-,  148 
Fila  lateralia  pontis,  148 
Fillet.     See  Lemniscus. 
Filum  durae  matris  spinalis,  74 
terminale,  74 

externum,  74 

internum,  74 
Fimbria  hippocampi,  250,  269 
Final  common  path,  94,  311 

re    or  fissura),  calcarine,  238,  292 
callosal.     See  Sulcus  of  corpus  callosum. 
callosomarginal,  240 
central,  of  Rolando,  233 
cerebri  lateralis,  2  5 1 
chorioid,  229,  251 
collateral,  239 

dentate.     See  Fissure,  hippocampal. 
development,  230,  231 
great  longitudinal,  2>2 

transverse.     See   Fissure,   transverse  cere- 
bral, 
hippocampal,  239,  269,  270 
lateral  cerebral,  233 


Fissure,  longitudinal  cerebral,  114,  232 

mediana, anterior,  of  medulla  oblongata,  119 
of  spinal  cord,  76,  82 
posterior,  of  medulla  oblongata,  119 

parieto-occipital,  239 

prima,  196,  199 

rhinal,  116,  240 

of  Rolando.     See  Sulcus,  central. 
-  la,  203 

Sylvian.     See  Fissure,  lateral  cerebral. 

transverse  cerebral,  215 
FleChsig,  direct  cerebellar  tract  of.     See  Tract, 

dorsal  spinocerebellar. 
Flexure,  cephalic,  31,  33 

cervical,  M,  33 

pontine,  31,  33 
Flocculus,  199 

Fluid,  cerebrospinal,  73,  126 
Folium  vermis,  198 
Foramen  caecum,  119 

interventricular,  26,  118 

of  Luschka.     See  Aperture,  lateral,  of  fourth 
ventricle. 

of  Majendie.     See  Aperture,  medial,  of  fourth 
ventricle. 

of  Monro.     See  Foramen,  interventricular. 
Forceps,  major,  245 

minor  (frontal  part  of  radiation  of  corpus  cal- 
losum). 
Fore-brain.     See  Prosencephalon. 
Forel,   fountain   decussation   of.     See  Decussa- 
tion, ventral  tegmental. 
Formatio  reticularis,  80,  136,  144 
Fornix,  270,  280 

body,  271 

columns,  271,  272 

commissure,  271,  280 

crura,  271 

fimbria,  270,  271 

longus,  282 
Fossa  interpedunculans,  115 

rhomboid,  126,  127 
Fountain  decussations  of  Forel  and  of  Mevnert, 

161,  167 
Fovea,  inferior,  127 

superior,  127 
Frenulum  veli  medullaris  anterior,  130 
Frog,  sympathetic  ganglia  of,  344,  345 
Funiculus,  95 

anterior,  76,  82 

cuneatus,  121,  137 

dorsal.     See  Funiculus,  posterior. 

gracilis,  121,  137 

lateralis,  76,  82 

posterior,  76,  82 

separans,  129 

teres.     See  Eminentia  medialis. 

ventral.     See  Funiculus,  anterior. 

Cjam.liated  cord.     See  Trunk,  sympathetic. 
Ganglion  or  ganglia,  autonomic.      See  Ganglia, 
sympathetic. 

basal,  252 

celiac,  349 

cerebrospinal  (sensory  ganglia  on  the  cerebro- 
spinal nerves),  38 

cervical,  inferior,  348 
middle,  348 
superior,  347 


I N  1  >  I  X 


hi,  i  iliarj ,  {51 
■  it  Corti.     See  Ganglion,  Bpiral. 
enteric,  small  ganglia  <it  myenteric  .m<l  Bub- 

mucous  plexuses,  351 
of  facial  nervi  mgHon,  geniculate. 

aerian.     See  Ganglion,  semilunar, 
geniculate,  192 
habenulae,  29,  220 
interpeduncular,  LIS,  164 
jugular,  193 
mesenteric,  3 19 
nodosum,  193 
otic,  151 
petrosal,  193 

of  Scarpa.     See  Ganglion,  vestibular, 
semilunar,  I'M 
Bensory,  $8 
sphenopalatine,  351 
Bpinal,  62 
development  of,  38,  40 
structure  of,  63,  64,  65 
spir.il.  185,  . 
submaxillary,  351 
sympathetic,  collateral,  335 
development  <»t',  41.  )  $5 
prevertebral.     See  Ganglia,  collateral  sym- 
pathetic. 
structure  of,  341 
of  sympathetic  trunk,  335 
terminal,  335 

vertebral.     See    Ganglia    of     sympathetic 
trunk, 
of  trigeminus.     See  Ganglion,  semilunar. 
vestibular,  188 
Gemmules,  43 
Geniculate  body.     See  Body. 

ganglion.     See  Ganglion. 
Gennari,  line  of,  283 
Genu  of  corpus  callosum,  243 
of  internal  capsule,  258,  262 
internum  of  facial  nerve,  175,  176,  180 
Glia-cells.     See  Cells,  neuroglia. 
Glial  sheath,  86 
Globus  pallidus,  254,  256,  324 
Glomeruli,  cerebellar,  208 
olfactory,  276 
of  sensory  axons,  63 
of  sympathetic  ganglia,  341 
Golgi  cells  of  Type  II,  44,  87 

method  of,  361 
Goll,  column  or  tract  of.       See  Fasciculus  gra- 
cilis. 
Gowers,  bundle  of.     See  Fasciculus  anterolater- 
al superficialis. 
Granular  layer  of  cerebellum,  208 
Gudden,  commissure  of,  227 
Gustatory  apparatus,  181 
Gyrus  (or  gyri),  angular,  236 
annectent,  234 
anterior  central,  235,  290 
ascending  parietal.     See  Gyrus,  posterior  cen- 
tral. 
breves  or  short  gyri  of  insula,  237 
callosal.     See  Gyrus  cinguli. 
centralis,  anterior,  235,  290 

posterior,  236,  292 
cinguli,  240 

dentatus.     See  Fascia  dentata. 
diagonal,  of  rhinencephalon,  267 


fornicatui 
frontal,  as<  ending 

tr.il. 

inferior, 

middle, 

superior,  2  $5,  240 
fusiform,  240 

hippocampal,  1 16,  240,  277 
iu-.iil.i-,  J  57 

limbic.     Se<   L  >be,  limbic, 
lingual,  239,  2<>2 
longus  insula;,  2  $7 

marginalis.  .  superior  frontal. 

olfactory,  lateral,  1 16,  266,  J77 

medial,  1 16,  266 
orbital,  241 

postcentral.     See  Gyrus,  posterior  central, 
posterior  cenl  ral,  _'  56,  2(>2 
precentral      5  ntral. 

reel  u>,  241 

subcallosus  (pedunculus  corporis  callosi),  267 
supracallosal,  244,  270 
supramaximal,  1  $6 
temporal,  inferior,  236 

middle,  2 

superior,  2  $6 

transverse,  236,  293 
uncinatus.     See  Gyrus,  hippocampal. 

HABENULA.     See  Xucleus  habenulae. 

Hearing,  organs  of,  185,  186  187,  309 

Heart,  innervation  of, 

II eat,  sensations  of,  105,  306 

Hemianopsia,  228 

Hemiplegia,  323 

Hemispheres,  cerebellar,  197,  198 

cerebral.     See  Cerebral  hemispheres. 
Hilus  nuclei  olivaris,  141 

Hind-brain.     See  Metencephalon  and  Rhomben- 
cephalon. 
Hippocampal  gyrus,  116,  240,  277 

commissure,  231,  271,  280,  296 
Hippocampus,  2^).  269,  277 
Histogenesis  of  cerebellar  cortex,  196 

of  cerebral  cortex,  230 

of  nervous  system,  37 

of  peripheral  nervous  system,  40 

of  spinal  cord,  38,  39,  42 
ganglia,  38,  40 
Horizontal  cells  of  Cajal,  285 
Horn     of     lateral     ventricle,     246.     (See    also 

Column.) 
Hypophysis,  222 

in  the  dogfish,  29 
Hypothalamus,  35,  222 

in  the  dogfish,  2(> 

pars  mamillaris,  222 
optica,  35 

[ncisura.     See  Notch. 

Indusium  griseum,  244,  270 
Infundibulum,  222 
Insula,  22'),  237 

Inter-brain.     See  Diencephalon. 
Interoceptor,  interoceptive,  66,  101 
Interpeduncular  fossa  ,115 

Interventricular  foramen,  2d,  118 
Intumescentia  cervicalis,  T 
lumbalis,  74,  84 


388 


IXDEX 


Island  of  Reil.     See  Insula. 
Iter   a   tertio   ad    quartum   ventriculum.      See 
Aqueductus  cerebri. 

Jelly-fishes,  19 

Joints,  sensory  fibers  of,  72 

KRAUSE,  end-bulb  of,  68 

Lamina  affixa,  215 

alar.     See  Plate,  alar. 

basal.     See  Plate,  basal. 

medullar  is  involuta.    See  Stratum  lacunosum. 

quadrigemina,  130,  158 

rostralis,  223,  243 

septi  pellucidi,  272 

terminalis,  25,  33,  223,  231 
Laminae  medullares  of  lentiform  nucleus,  254 

thalami,  216 
Lancisi,  nerve  of.     See  Stria  longitudinalis  me- 

dialis. 
Lateral  line  organs,  356 
Layers  of  cerebellar  cortex,  208 

of  cerebral  cortex,  286,  287 

ependymal,  37 

mantle,  37,  42,  196 

marginal,  37,  42,  196 

of  retina,  225 
Lemniscus,  lateral,  130,  157,  163,  165,  166,  186, 
1S7,  309 

medial,  135,  138,  145.  153,  163,  219,  313 

spinal.     See  Tract,  spinothalamic. 

trigeminal.     See  Path,  secondary  afferent,  of 
trigeminal  nerve. 
Ligamentum  denticulatum,  74 
Limen  insulae,  237,  268 
Line  (or  lines)  of  Baillarger,  283 

of  Gennari,  283 
Linea  splendens,  74 
Lingula  of  cerebellum,  197 

Lissauer,  tract  of.     See   Fasciculus  dorsolater- 
al. 
Lobe  (lobus  or  lobes)  of  cerebellum,  197,  198, 
200,  201,  202 

of  cerebrum,  234 

frontal,  234 

inferior,  28 

insular.     See  Insula. 

limbic.     See  Gyrus  fornicatus. 

linear  lateralis,  27 

occipital,  236,  238 

olfactorv,  267 

optic,  27,  28,  165 

parietal,  236 

pyriform.     See  Area,  pyriform. 

temporal,  235 

visceral,  27 
Lobule  for  lobulus)  ansiformis,  201 

bi  venter,  199 

centralis,  197 

paracentral,  240,  290 

paramedian  us,  201 

parietal,  inferior,  236 
superior,  236 

postcentral.     See  Gyrus  longus  insulae. 

precentral.     See  Gyri  breves  insulae. 

quadrangularis,  198 

quadrate.     See  Precuneus. 

semilunaris,  inferior,  198 


Lobule  semilunaris,  superior,  198 

simplex,  200 
Localization  of  function  in  cerebellum,  202 
in  cerebral  cortex,  290 
in  thalamus,  219 
Locus  caeruleus,  128 
Luschka,  foramen  of.     See  Aperture,  lateral,  of 

fourth  ventricle. 
Luys,  nucleus  of.     See  Nucleus  hypothalamics. 
Lyra.     See  Commissure,  hippocampal. 

Mackosmatic  mammals,  265 

Magendie,  foramen  of.    See  Aperture,  medial,  of 

fourth  ventricle. 
Mammillary  body,  222,  280 
Mantle.     See  Cerebral  cortex. 

layer.     See  Layer. 
Marchi  stain  for  degenerated  nerves,  360 
Martinotti,  cells  of,  285 
Massa  intermedia,  216 
Matter,  central  grav,  136,  158 
grav,  42,  79,  87 
white,  42,  79,  86 
Medulla  oblongata,  114,  118 
closed  portion  of,  119 
development,  35.     (See  also  Myelencepha- 

lon.) 
in  the  dogfish,  26,  27,  28 
fissure,  anterior  median,  119 

posterior  median,  119 
form,  118,  119,  120,  121,  122 
gray  matter,  136 
internal  structure,  132 
length,  118 

motor  nuclei,  170,  174 
open  portion  of,  119 
sensorv  nuclei,  180,  182 
sulci,  119 
spinalis.     See  Spinal  cord. 
Meissner,  corpuscles  of,  68 

plexus  of,  351 
Meninges,  73,  74 
Merkel,  corpuscle  of,  68 
Mesencephalon,  129,  158 
development,  24,  31,  35,  36 
in  the  dogfish,  27,  28 
form,  129 

internal  structure,  158 
Metamerism,  58.     (See  also  Segmentation.) 
Metathalamus,  220 
Metencephalon,  31,  33,  36 
Meynert,  fasciculus  retroflexus  of,  220 

fountain    decussation    of.     See    Decussation, 
dorsal  tegmental. 
Microsmatic  mammals,  265 
Mid-brain.     See  Mesencephalon. 
Mitochondria,  49 
Molecular  layer  of  cerebellum,  208 

of  cerebral  cortex,  286 
Monakcw,  bundle  of.     See  Tract,  rubrospinal. 
Monro,  foramen  of.     See  Foramen,  interventric- 
ular. 
Monticulus,  198 

Moss-fibers  of  cerebellum,  209,  210 
Motor  apparatus,  316 

area  of  cerebral  cortex,  290,  317,  318 
end-plate,  62 
Muscle,  branchial,  174 

cardiac,  innervation  of,  353 


I\IM   \ 


Muscle  ol  eyeball,  innervation  of,  352 

ol  facial  expression,  innervation  of,  192 

ol  larj  n\,  in  in  i  \  .ii  ion  of,  I'M 

ol  111,1-1  ical  ion,  innei  vati<  in  of,  1 92 

mi  \  i  endings  in,  '>-',  72 

sense  I  propnocept  ive),  72,  ''',.  100,  $11 

skeletal.     See  Muscle,  lir.inclii.il  and  somatic. 

smooth  or  unstriated.     Sec  Muscle,  visceral. 

somal  ii\  innei  \  al  ion  of,  62,  1 70 

Btriated.     See  Muscle,  branchial  and  somatic 

ut  tongue,  innen  al  ion  of,  I'M 

\  is  :eral,  innen  .ii  ion  of,  '>1 ,  1  74,  1 77 
Muscle-spindles,  72 
Myelencephalon,  31,  32,  33,  36 
Myelin,  46 

sheal  h.     See  Sheath. 
Myelination  in  cerebral  cortex,  2Sl> 

in  spinal  cord,  1 1  2 
Myotome,  58,  1  70 

\i  OPAl  i  u  m,  1  !(•,  2M,  242 

Neol  halamus,  219 

Nerve  (Nervus),  abducens,  123,  154,  175,  192 

accessory,  125,  176,  177,  194 

acoustic,  123,  185,  192 

auditory.     See  Nerve,  acoustic. 

cardiac,  348,  349 

cerebrospinal,  56 

chorda  tympani,  1(>2,  352 

ciliary,  552 

cochlear,  149,  185,  193 

components,  61.     (See  also  Nerve-fibers.) 

cranial,  56,  152,  155,  168 

facial,  125,  155,  175,  192 

glossopharyngeal,  125,  L93 

hypoglossal,  125,  175,  194 

intermedius,  125,  162 

of  Lancisi.     Sec  Stria  longitudinalis  medialis. 

lingual,  192 

oculomotor,  150,  1(>4,  171,  172,  I'M 

olfactory,  191,  2()^< 

optic,  191,  22'^ 

phrenic,  59 

pneumogastric.     See  Nerve,  vagus. 

spinal,  5(i,  58,  65 
development  of,  40 

splanchnic,  548 

sympal  hetic,  545 

terminalis,  27,  190 

thoracic,  ^s 

trigeminal,  124,  154,  174,  1S2,  191 

trochlear,  125,  165,  175,  191 

vagus,  125,  17s,  193 

vestibular,  14'),  185,  193,  514 

of  Wrisberg.     See  Nervus  intermedius. 
Nerve-cells,  45.      (See  also  Neurons  and  Cells.) 

autonomic.     See  Neurons,  sympathetic. 

motor,  lor  involuntary  muscles,  177 
for  voluntary  muscles,  177 

processes,  45 

shape,  4  5 

struct  lire,  47 

types  of,  45,  44 
Nerve-endings,  encapsulated,  68 

free  in  epidermis,  (>7 

in  free  arborizations,  67 

in  hair-follicles,  7<i,  7  1 

in  Meissner's  corpi 

on  Merkel's  touch-cells,  68 


Nervi  endings  in  muscle-spindles,  71,  1 '. 
in  Pacinian  corpu 
pei  ipheral,  66  72 

in  -\  nap 

Mill,   corpuscli 
in  tendon 
in  \  oluntarj  mus<  I' 

fibers,  I  - 
afferent,    58,    6 

mal  ic  and  \  i-i  ii. d  affen 
autonomii         Sei  .  lionic 

and  postganglionic. 

i  ,!   i  i  ol  nil. ii    ,.,11,  y,   206 

of  cerebral  <  ortex,  2s.i 

i  lassifii  .0  inn  nl.  oti 

collateral,   13,  ''7 

degeneration  of,  52,  105,  t06%  1>>7 

dc\  elopmenl ,  40,  1 1 

of  dorsal  rout ,  (>5 

efferent,  58 

exterocepl  ive,  66 

gray.     See  Nerve-fibers,  postganglionic, 

interocepl  i\  e,  66 

to  involuntary  muscles,  61 

medullated.     See  Nerve-fibers,  myelinated. 

motor,  59,  62,  94 

myelinated,  45,  46,  47,  63,  66,  67,  87 

non-medullated.     See  Nerve-fibers,  unmyelin- 
ated. 

postganglionic,  557,   i  I  j 

preganglii inic,  557,  544 

primary  motor,  62,  90 

propriocepl  ive,  66,  72 

regeneration,  r<2 

nl  Remak.     See  Nerve-fibers,  unmyelinated. 

soni.it  ic  afferenl  ,61,  66 

g<  neral,  168,  182,  162,  193 
special,  168,  I'M,  193 
efferent,  61,  (.2,  168,  I'M,  1<>2,  194 

sympathetic.       See   Nerve-fibers,   postgangli- 
onic. 

unmyelinated,  47,  63,  66,  67,  87,  98,  104 

visceral  afferent,  (d 

general,  168,  181,  193,  555 

special,   U.S.    ISO,    [92,    193 
efferent,  (>1 

general,  168,  178,  192,  193,  194,  336 
special,  168,  17  1,  192,  193,  I'M 
to  voluntary   muscles.     See   Nervi 

matic  efferent  and  special  visceral  efferent. 
of  white  rami,  ol  ,5  17 

substance  of  brain  and  cord,  47 
Nerve-root.     See  Root. 
Nervous  system,  autonomic,  55') 
cranial,  .^V) 
craniosacral,  540 
sa<  ral,  ^^^ 

t  horacicolumbar,  .^.^'K  540 
central,  20,  21.  56,  57 
cerebrospinal, 

development  of,  24,  >2,  36,  J7 
diffuse.  18,  1",  540 
invertebrate,  1",  20,  21.  22 
peripheral,  56 
subdivisions  ol, 
sympathetic,  56 
vertebrate,  21, 
Net,  nervous,  19,  3 


39° 


INDEX 


Neural  crest,  37 

groove,  24,  3 1 

tube,  24,  31,  36 
Neurilemma,  41,  46,  47 

Neurobiotaxis,  179 
Neuroblasts,  37,  39 
Neurofibrils,  48,  (9,  50 
Neuroglia,  85,  86 
Neuromuscular  end-organ,  72 

mechanism,  17 
Neuron  or  neurons,  43.     (See  also  Nerve-cells.) 

basket  cell,  50 

bipolar,  39,  44,  63 

chains,  43,  4",  53,  54 

concept,  ^2 

of  cerebellar  cortex,  207,  208,  209 

of  cerebral  cortex,  285 

development  of,  37 

form  of,  42 

horizontal,  of  Cajal,  285 

interrelation  of,  49 

lower  motor,  318 

of  Martinotti,  285 

motor,  22,  44,  46,  177 

multipolar,  44 

of  olfactory  bulb,  275 

polarization  of,  50 

postganglionic,  337 

preganglionic,  337,  339,  341 

of  Purkinje,  207 

pyramidal,  43,  44,  285 

of  retina,  225,  226 

sensory,  22,  23,  37,  63 

stellate,  285 

structure  of,  47 

sympathetic,  341 

theory  of.     See  Neuron  concept. 

tvpe  I,  44,  87 

type  II,  44,  45,  87,  88 

unipolar,  39,  44,  63 

upper  motor,  317 
Neuropil,  20,  21 
Neuropore,  31 

Nissl  bodies  or  granules,  48,  51 
Nodes  of  Ranvicr,  47 
Nodule  of  vermis,  198 

Non-medullated  fibers.    See  Nerve-fibers,  unmye- 
linated. 
Notch,  anterior  cerebellar,  197 

posterior  cerebellar,  197 

preoccipital,  234 
Nucleated  sheath.     See  Neurilemma. 
Nucleus  (or  nuclei)  of  abduccns  N.,  154,  173 

accessory  cuneate,  138 

of  accessory  N.,  I'M 

of    acoustic    N.     See    Nuclei,    cochlear    and 
vestibular. 

ambiguus,  146,  176 

amygdaloid,  249,  257 

anterior  thalami,  217,  218 

arcuate,  140,  143 

arcuatus  thalami,  218 

of  Bechterew,  152,  189 

caudatus,  253 

centralis,  superior,  1 57 
of  thalamus,  218 

of  cerebellum,  203,  204 

cochlear,  123,  149,  185 

commissural,  330 


Nucleus  of  corpus  mamillare,  222 
cuneatus,  122,  134,  137,  139 
of  Darkschewitsch,  153 
of  Deiters,  151,  189 
dentatus,  203,  206,  211 
dorsalis,  90,  100 
of  dorsal  funiculus.     See  Nucleus  gracilis  and 

Nucleus  cuneatus. 
dorsal  motor,  of  vagus,  146,  178 

thalamic.     See  Nucleus,  anterior  thalami. 
of  Edinger  and  Westphal,  178 
emboliformis,  203 
external  round,  138 
of  facial  N.,  motor,  153,  175,  179 
of  fasciculus  cuneatus.    See  Nucleus  cuneatus. 

gracilis.     See  Nucleus  gracilis. 

solitarius.    See  Nucleus  of  tractus  solitarius. 
fastigii,  204,  211 

of  fifth  nerve.     See  Nuclei  of  trigeminal  nerve, 
of  fourth  nerve.       See  Nucleus  of  trochlear 

nerve, 
funiculi  cuneati.     See  Nucleus  cuneatus. 

gracilis.     See  Nucleus  gracilis, 
globosus  of  cerebellum,  203 

of  thalamus,  218 
of  glossopharyngeal  nerve.     See  Nucleus  am- 
biguus and  Nucleus  of  tractus  solitarius. 
of  Goll.     See  Nucleus  gracilis, 
gracilis,  122,  134,  137 
habenula;,  29,  220 
of  hypoglossal  nerve,  145,  173 
hypothalamicus  (Corpus  Luysi),  223 
of  inferior  colliculus,  165 
internal  round  nucleus,  138 
interpeduncular,  115,  164 
interstitial,  153 
of  lateral  lemniscus,  157,  187 
lateral  reticular,  of  medulla  oblongata,   143, 

145 
lateral  thalamic,  217,  219 
lemnisci  lateralis,  157,  187 
lenticular,  254         ^ 
lentiform,  254 

of  Luys.     See  Nucleus  hypothalamicus. 
of  medial  longitudinal  fasciculus,  153 
medial  thalamic,  217,  218 
mesencephalic.    See  Nucleus  of  trigeminal  N. 
motor,    of   tegmentum    (motorius  tegmenti), 

145,  161 
of  nerve-cell,  47 
of  oculomotor  N.,  164,  171 
olivary,  141,  142 

accessory,  142 
dorsal,  142 
medial,  142 

inferior,  141 

superior,  151,  186 
of  origin,  180 
pontis,  148,  149 

radicis  descendentis   N.  tngemini.      See  Nu- 
cleus of  tractus  spinalis  of  N.  V. 
red,  159,  160 

roof,  of  cerebellum.     See  Nucleus  fastigii. 
ruber.     See  Nucleus,  red. 
salivatory,  178 

of  Schwalbe.     See  Nucleus,  medial  vestibular, 
semilunar,  of  thalamus,  218 
of  sixth  nerve,  154,  173 
somatic  afferent,  182,  185 


IMH   \ 


Nucleus,  somatit  efferenl ,  1 7o 
of  spinal  tra<  I  N    \ ,  136,  144,  155,  182 
tecti.     Sec  Nm  Ir its  fast  igii. 
tegmental,  doi  sal,  158 

ventral,  158 
terminal,  180 
thalamic,  217,  218 
ol  trad  us  solitarius,  1  l<>,  181,  330 

spinalis  V  trigemini,  1 36,  lit,  145,  1 S2 
i  »l  i  rapezoid  body,  1 86 
of  i rigeminal  \.,  IS  I,  156 
main  sensory,  1  55,  182 
mesencephalic,  155,  184 
motor,  1  55,  1  74 
spinal,  136,  144,  155,  182 
of  trochlear  V.  163,  173 
ot  vagus,  motor.     Sec  Nucleus,  dorsal  motor, 
of  vagus  and  Nucleus  ambiguus. 
sensory.     Sec  Nucleus  of  tract  us  solitarius. 
\vnt  i.i  1  i  halamic,  2 1 8 
vestibular,  151,  188 
viscera]  afferent,  180 
efferent,  174,  177 

Obex,  129 

Olfactory  apparatus,  274-282 
bulb,  265,  274 

cells  of  nasal  mucous  membrane,  274 

cortex,  277,278,279.    (See also ArchipalUum.) 

glomeruli,  276 

gyri,  116,  266,  277 

lobe,  267 

nerve,  265,  275 

roots.     See  Gyri,  olfactory. 

striae,  266,  277 

tract,  265,  277 

trigone,  266 

tubercle,  268,  282 
Olive  (oliva,  olivary  body),  121 

accessory,  142 

inferior,  141 

superior,  151,  186 
Opercula,  230,  237 
Opt  ic  apparatus,  225 

chiasma,  223,  226 

cup,  32,  33,  225 

lobes,  27,  1$,  165 

nerve,  225,  226 

radiation,  227 

tectum.     See  Colliculus,  superior. 

tract,  226 

vesicle,  225 
Organ  of  Corti,  185,  186 

lateral  line,  356 

spiral,  185,  186 

Pacinian  corpuscles,  69 

Pain,  apparatus  of,  68,  103,  105,  306 

Palseothalamus.     See  Thalamus,  old. 

Pallium,  25,  32,  33,  229 

Para  flocculus,  202 

Paralysis,  322,  323 

Paraphysis,  31 

Parasympathetic  system.     See  Nervous  system, 

craniosacral  autonomic. 
Pars  anterior  lobuli  quadrangularis,  198 

basilaris  pontis,  124,  147 

dorsalis  pontis,  124,  149 

frontalis  capsular  internae,  258,  259 


Pars  intermedia  of   Wrisb  u  in- 

termedius. 
niaiiiill.ii  is  hj  poi  halami,  222 

ipitalis  i  apsulae  intei  nae, 
opt  ica  h\  po1  nalami,  35 
postei  ior  lobuli  quadrangulai  i 
Path  (or  pathwaj  I,  aff<  rent  i  eretx  liar,  11  \,  J14 

spinal,  91 

auditory,  186,  309 
•  erebello  rubra  spinal,  $26 
cortico-ponto  i  ei  ebellai ,  1  19, 
i  raniosai  ral,  352,  $5  J,  >;  1 
efferenl  ,216 
for  eye,  ^2 

for  Ileal  t  ,353 

for  stomach,  353 

for  submaxillar)  gland,  M^2 

for  urinal  \  bladder,  $5  I 
exteroceptive,  66,  101,  102,  502 
e\t  rapyramidal  motor,  524 

final  n n,  9  I,  311 

motor,   109,  216 

for  cranial  nen es,  520 
for  spinal  nerves,  319 

for  muscular  sense.    See  Path,  proprioceptive. 

olfa y,  280 

for  pain',   103,   104,   105,  500 

proprioceptive,  72,  99,  100,  311 

secondary  afferent,   from    tractus    solitarius, 
181 
of  trigeminal  N.,  163,  183,  185,  507 
vestibular,  190 

for  thermal  sensibility,  105,  500 

thoracicolumbar,  552,  555,  554 

for  touch,  101,  102,  505 

vestibular,  190 

visual,  226,  227,  228,  310 
Peduncle   (or  peduncles),  cerebellar,   204,   2^5, 
206,  211 

cerebral,  129,  15S 

of  corpus  callosum.     See  Cyrus  subcall 

of  mammillary  body,  222 

olivary.     See  Stalk  of  superior  olive. 

of  pineal  body.     See  Stalk  of  pineal  body. 
Perforated  space,  anterior.     See  Substantia  per- 
forata anterior. 
Perikaryon,  43 

Pes  pedunculi.     See  Basis  pedunculi. 
Pia  mater,  75 
Pine  al  body.   150 

Pituitary  body.     See  Hypophysis. 
Plate,  alar,  34,  42,  194 

basal,  54,  42 

neural,  24 

roof,  of  prosencephalon,  213 
Plexus  of  Auerbach,  351 

brachial,  58 

cardiac",  349 

celiac,  54(> 

chorioid,  lateral,  251 
of  fourth  ventricle,  128 
of  third  ventricle,  22^ 
'  esophageal,  549 

gastric,  349 

hypogastric,  351 

intercellular,  of  sympathetic  ganglion,  544 

lumbosacral. 

Meissner's,  351 

mesenteric,  349 


392 


INDEX 


Plexus,  myenteric,  351 

pelvic,  351 

pericellular,  of  spinal  ganglion,  65 
of  sympathetic  ganglion,  345 

pulmonary,  540 

solar,  349 

submucous,  351 

sympathetic,  345,  348 

vesical,  354 
Polarity  of  the  neuron,  50 
Poles  of  cerebral  hemisphere,  232 
Pons  (Varoli),  114,  12.5 

basilar  or  ventral  part  of,  124,  147 

dorsal  or  tegmental  part  of,  124,  149 

form,  123 

internal  structure,  147 

longitudinal  fasciculi,  147 

nuclei  of,   14S 

taenia  of,  148 

transverse  fibers  of,  147 
Ponticulus.     See  Tcenia  of  fourth  ventricle. 
Portio  major  X.  trigemini,  125 

minor  \.  trigemini,  125 
Postganglionic  fibers,  337,  343 
Precuneus,  240 
Preganglionic  fibers,  337,  344 
Pressure,  apparatus  of  sensibility  to,  66 
Presubiculum,  277 
Processus  reticularis.    See  Reticular  formation  of 

spinal  cord. 
Projection  centers,  290 

fibers,  297 
Proprioceptor,  proprioceptive,  72,  99,  100,  183, 

185,  311 
Prosencephalon,  24,  25.  31,  36,  113 
Protoplasm,  17 

Psalterium.     See  Commissure,  hippocampal. 
Pulvinar,  214,  217,  227 
Purkinje,  cells  of,  207 
Putamen,  254,  255 
Pyramid  (or  pyramis)  of  cerebellum,  198 

of  medulla  oblongata,  119,  136 

of  vermis,  198 
Pyriform  lobe,  116,  268,  277 

Radiation  (or  radiatio),  auditory  or  acoustic, 
261 

of  corpus  callosum,  243,  245 

occipitothalamica.     See  Radiation,  optic. 

optic,  227,  261,  264 

sensory,  264 

thalamic,  216,  217,  260,  263 

thalamotemporal,  264 
Radix    descendens     (mesencephalica)     N.    tri- 
gemini.    Sec  Root,  mesencephalic  X.  Y. 

X.  facialis,  175 
Ramus  communicans,  335,  346 
grav,  337\  347 
white,  335,  347 

dorsal,  58 

ventral,  58 
Ranvier,  constrictions  or  nodes  of,  47 
Receptor,  10,  53,  91 
Recess,  lateral,  of  fourth  ventricle,  125 

lateralis  fossae  rhomboidese,  125 

optic,  223 

pineal,  221 

suprapineal,  221 
Reflex  act,  91 


Reflex  arc,  20,  53,  91,  92,  93,  327 
auditory,  331 

of  brain  stem,  328,  M'>,  330,  331,  532 
for  coughing  and  vomiting,  330 
of  medulla  oblongata,  MX,  329,  330 
myenteric,  340 
optic,  .^M 
pupillary,  332,  333 
respiratory,  330 
scratch,  94 

of  spinal  cord,  91,  92,  93,  94,  328 
vestibular,  328,  329 
visceral,  340 
Regeneration  of  nerve-fibers,  52 
Reil,  island  of.     See  Insula. 
Respiratory  apparatus,  330 
Restiform  body,  122,  143,  205 

medial  part  of,  205 
Reticular  formation  (or  substance),  80,  136,  144 
Retina,  225 

Rhinencephalon,  25,  32,  115,  265 
Rhombencephalon,  25,  31,  32,  35,  36,  113 
Rhombic  lip,  195 
Rod  and  cone  cells,  226 
Rolando,  fissure  of.     See  Sulcus  centralis, 
substantia  gelatinosa  of,  80 
tubercle  of.     See  Tuberculum  cinereum. 
Root  of  abducens  nerve,  123 
of  accessory  nerve,  76,  123 
of  acoustic  nerve,  123 
anterior  spinal.     See  Root,  ventral, 
dorsal,  58,  76,  95,  96,  07 
of  facial  nerve,  123 
field.     See  Sensory  root  field, 
of  glossopharyngeal  nerve,  123 
of  hypoglossal  nerve,  123 
mesencephalic,  N.  V.  155,  156 
of  oculomotor  nerve,  130 
posterior,  spinal.     See  Root,  dorsal, 
spinal,  78 

of  trigeminal  nerve,  124,  125 
of  trochlear  nerve,  191 
of  vagus  nerve,  123 
ventral,  58,  76 
Rostrum  of  corpus  callosum,  243 
Rudiment  of  hippocampus,  244,  267,  270 

Saccule,  193 

Saccus  vasculosus,  28,  29 

Scarpa,  ganglion  of.     See  Ganglion,  vestibular. 

Schultze,  comma-tract  of,  97,  107 

Schwalbe,  vestibular  nucleus  of.      See  Nucleus, 

medial  vestibular. 
Schwann,  sheath  of.     See  Neurilemma. 
Sea-anemones,  17,  19 
Segmentation  of  spinal  cord,  74 
Semicircular  canals,  193 

Septomarginal  bundle  or  fasciculus,  97,  107 
Sensation  (or  sensibility)  of  cold,  105,  306 

of  hearing,  185,  186,  187,  309 

of  heat,  105,  306 

muscular,  72,  99,  100,  311 

of  pain,  68,  103,  105,  306 

of  pressure,  303 

of  sight,  225,  228 

of  smell,  265 

of  taste,  181 

of  touch,  66,  77,  101,  303 

visceral,  336 


IM)I   \ 


393 


Sensory  rool  field,  59,  60 
Sept  urn  pellucidum,  243,  272 
\x istenor  intermediate,  83 
median,  83 

DOBl  iriiin,  74 
Sliark.     Sec  Dogfish. 
Sheath,  glial,  86 
medullary.    See  Sheath,  myelin, 
myelin,  1 1 ,  16,  47 
of  Schwann.     See  Neurilemma. 
si-iii ,  organs  of,  -'25  228 
Smell,  organs  of,  26S  282 
Solitary  bundle.     See  Tractus  solitarius. 
Somesl he! ic  area,  2l>2 
Speech,  apparal  us  of,  295,  296 
Spider-cells,  86 
Spinal  cord,  56,  72,  75 

cervical  enlargement,  73,  79,  84 
characters  ol  different  regions,  83 
columns  of  gray  matter,  79 
of  white  matter.     Sec  Funiculus. 
of  cells.     See  Cell-columns. 
commissures,  80 
coverings,  73 
corrua.     See  Columns. 
degenerations  from  brain  lesions,  105,  106 
from  cord  lesions,  105,  106 
from  sect  ion  of  dorsal  roots,  106 
development,  41 ',  42 
in  fetus  and  infant,  77 
fissure,  anterior  median,  76 
funiculi,  82 
glial  sheath,  86 

gray  matter  or  substance,  78,  79,  80,  81,  87 
cell-columns,  89,  90 
columns,  79 
horns.     See  Columns. 
microscopic  structure,  87 
nuclei.     See  Cell-columns. 
relation  to  size  of  nerves,  84 
horn.     See  Column. 
internal  structure,  85 
lumbar  enlargement,  74,  SI,  84 
microscopic  structure,  85 
relation  to  vertebral  canal,  77 
reflex  mechanism  of,  91,  92,  93 
sacral  region,  74,  81,  84 
segmentation,  74 
sulcus,  anterolateral,  76 
posterior,  76 

intermediate,  76 
posterolateral,  76 
thoracic  region,  80,  84 
tracts,  95-112,  110 
white  matter  (or  substance),  81,  86 
area  in  different  regions,  82 
microscopic  structure,  86,  87 
ganglion.     See  Ganglion. 
nerve.     See  Nerve. 
Spiracle,  356 
Splanchnic  nerves,  348 
Splenium  corporis  callosi,  244 
Spongioblasts,  37 
Stalk,  optic,  32,  225 
of  pineal  body,  221 
of  superior  olive,  151,  175 
Stomach,  innervation  of,  353 
Stratum  griseum  centrale,  163 
of  superior  colliculus,  167 


Str.it urn  lacunosum, 

lemni -.i  i,   Id, 

lui  idum,  27n 

opticum,  167 

oriens,  279 

profundum,  166,  W>7 

radiatum,  27u 

/on. ilc  of  superior  i  olliculus,  \<>7 

ol  t  halamus,  216 
Stria  (or  striae)  acustica.    See  Stria  medullarea 

acusl  ica. 
of  Baillarger, 
of  ( iennari,  283 
longil  udinalis  lateralis,  245,  270 

medialis,  2  15,  270 
medullaris  acusl  ica,  1  23,  1 27,  1  *<> 

thalami,  215,  220,  281 
olfactoria  lateralis,  266,  277 

medialis,   266 
semicin  ularis.     See  Stria  terminalis. 
terminalis,  214,  281 
Stripe  of  baillarger,  283 

of  (iennari,  283 
Subarachnoid  space,  73 
Subiculum,  277,  280 
Substantia  alba,  42,  79,  86 
ferruginea,  128 
grisea,  42,  79,  87 

centralis,  136,  158,  163 
gelatinosa,  Rolandi,  80 
centralis,  86 

externa.     See  Sheath,  glial, 
nigra,  129,  158,  164 
perforata,  anterior,  267,  282 

posterior,  1  1  5 
reticularis.     See  Reticular  formation. 

alba,  144 

grisea,  145 
Subthalamic  tegmental  region.     See  Subthala- 

mus. 
Subl  halamus,  222 
Sulcus  (or  sulci),  anterior  lateral,  76,  119 

parol  factory,  239 
basilar,  124 

callosal.     See  Sulcus  of  corpus  callosum. 
central,  of  Rolandi,  233 
cerebellar,  199 
cerebral,  233,  235,  236,  239 
cinguli,  239 
circularis  insula?,  237 
of  corpus  callosum,  239 
cruciate,  1 14 
frontal,  inferior,  235 

middle,  235 

superior,  235 
horizontals  cerebelli,  197 
hypothalamicus,  ll-~> 
insulae,  23  7 

intermedins,  posterior,  76,  127 
intraparietal,  236 
lateral,  of  mesencephalon,  130 
lateralis,  anterior.  See  Sulcus,  anterior  lateral. 

posterior.      See  Sulcus,  posterior  lateral. 
limitans,  34,  42,  129 

instihe.     See  Sulcus  circularis  insulae. 
lunatus,  237 

medianus  posterior  of  spinal  cord,  76 
of  medulla  oblongata,  1  19 
occipitalis  transversus,  236 


394 


INDEX 


Sulcus  of  oculomotor  nerve,  130 
olfactory,  241 
orbital,  241 
paracentral,  239 
parolfactorius,  anterior,  239 

posterior,  267,  239 
postcentral,  inferior,  236 

superior,  236 
postclivalis,  1(>7 
posterior  lateral,  76,  1 19 

parolfactory,  239,  267 
precentral,  235 

inferior,  23S 

superior,  235 
prepyramidal,  202 
primarius.     Sec  Fissura  prima. 
rhinalis.     See  Fissure,  rhinal. 
of  spinal  cord,  76 

subparietal,  239 

temporal,  inferior,  236 
middle,  236 

superior,  236 
uvulo-nodularis,  203 

Sylvius,  aqueduct  of,  26,  158 

fissure  of,  233 
Sympathetic  ganglia.     See  Ganglion. 

system,  50,  57,  334 
Synapse,  49,  50,  51,  55 
Syncytium,  38 
System.     See  Nervous  system. 

Tactile  corpuscles,  68 
Taenia  chorioidea,  214 

of  fourth  ventricle,  126 

pontis.     See  Fila  lateralia  pontis. 

teeth     See  Stria  longitudinalis  lateralis. 

thalami,  214,  224 

ventriculi  quarti,  126 
Tapetum,  245 
Taste,  apparatus  of,  181 
Tectum  mesencephali,  28,  165 
Tegmentum,  12°,  1 58 
Tela  chorioidea  of  fourth  ventricle,  128 

of  third  ventricle,  215,  224 
Telencephalon,  36 

development,  25,  31,  32,  33 

in  the  dogfish,  27,  28 

medium,  212,  229 
Temperature,  apparatus  of,  105,  306 
Tendon,  nerve  endings  in,  72 
Tentorium  cerebelli,  113 
Thalamencephalon.     See  Diencephalon. 
Thalamus,  213 

development,  35,  213 

in  the  dogfish,  29 

ending  of  sensory  tracts  in,  219 

lamina,  external  medullary,  216 
internal  medullary,  216 

new,  219 

nuclei,  217 

old,  218 

pulvinar,  218 

radiation  of,  216,  217,  260,  263 

stalks,  263 

stratum  zonalc,  216 

thalamocortical  fibers,  263 

tubercle,  anterior,  213 
Tigroid  bodies.     See  Nissl  bodies. 
Tonsil  (tonsilla  cerebelli),  199 


Touch,  apparatus  of,  66,  71,  101,  303 

Tract  or  tracts,  95.      (See  also  Bundle  and  Fas- 

I  It  III  US.  I 

bulbospinal,  1 1 1 

of  Burdach.     See  Fasciculus  cuneatus. 

central  sensory.     See  Path. 

cerebellobulbar.     See  Tract,  fastigiobulbar. 

cerebellotegmental,  211,  212 

comma,  97,  107 

corticobulbar,  165,  260,  321 

corticopontine,    147,    164.      (See  also   Tracts, 

frontopontine  and  temporopontine.) 
corticorubral,  161,  260 
corticospinal,  109,  133,  136,  147,  165,  260,  320 

lateral,  1()(),  134,  136 

ventral,  134,  136 
corl  icothalamic,  263 

direct  cerebellar.    See  Tract,  dorsal  spinocere- 
bellar. 
dorsal  spinocerebellar,  110,  143,  144,  145,  205 
efferent,  from  cerebellum,  211 

from  cerebral  hemisphere,  297 

from  mesencephalon.       See   Tracts,  tecto- 
spinal, tectobulbar,  and  rubrospinal, 
fastigiobulbar,  212 

of  Flechsig.    See  Tract,  dorsal  spinocerebellar, 
frontal  olfactory  projection,  281 
frontopontine,  164,  259 
of  Goll.     See  Fasciculus  gracilis, 
of  Gowers.    See  Tract,  ventral  spinocerebellar, 
habenulo-peduncular.      See  Fasciculus  retro- 

rlexus. 
of  Helweg.     See  Tract,  bulbospinal, 
lateralis  minor.  See  Fasciculus  lateralis  minor, 
of  Lissauer.     See  Fasciculus  dorsolateral, 
mamillotegmental,  222,  281 
mamillothalamic,  217,  222 
mesencephalic,  of  N.  V.       See  Root,  mesen- 
cephalic, N.  V. 
of  Meynerit.     See  Fasciculus  retroflexus. 
of  Monakow.     See  Tract,  rubrospinal, 
nucleocerebellar,  205 
olfactory,  265,  277 

olivocerebellar.     See  Fibers,  olivocerebellar, 
olivospinal.     See  Tract,  bulbospinal, 
optic,  226 

pontocerebellar.     See  Brachium  pontis. 
pontospinal.     See  Tract,  reticulospinal, 
predorsal.     See  Tract,  tectospinal, 
prepyramidal.     See  Tract,  rubrospinal, 
projection,  297 
pyramidal,  109 

aberrant,  321 

direct,  109 

crossed,  109 

uncrossed  lateral,  320 
reticulospinal,  160 
rubroreticular,  160,  161 
rubrospinal,  of  Monakow,  110,  145,  161 
of  Schultze,  107 
septomarginal,  97,  107 
solitariospinalis,  330 
solitary  (solitarius),  132,  181,  330 
spinal,  of  N.  V.,  132,  136,  144 
of  spinal  cord,  94-112,  110 
spinocerebellar,  dorsal,  100,  314 

ventral,  100,  313 
spino-olivary,  105 
spinotectal,  105,  145 


INDEX 


395 


Tract,  spinothalamic,  1 15,  163,  219,  307 
lateral,  102 
ventral,  101,  305 

Btrionigral,  1<>1,  263 

sulcomarginal,  108 

tei  tobulbar,  161,  167 

tectocerebellar,  206 

tectospinal,  ill,  145,  161,  167 

tegmentospinal.     Sec  Tract,  reticulospinal. 

temporoponl ine,  164,  261 

i  halamocon  ical,  26 1 

i  halamo-olivai  j ,  115,  219 

t  halamospinal,  21(> 

transverse  peduncular,  369 

trigeminothalamic,  183,  185 

ventral  spinocerebellar,  100,  144,  145,  157,206 

vestibulocerebellar,  190,  206 

vestibulospinal,  111,  190,  M{> 

of  Vicq  d'Azyr.     Sec  Tract,  mamillothalamic. 
Trapezium.     Sec  Trapezoid  body. 
Trapezoid  body,  121,  150,  186 
Triangle  of  Gombault  ami  Philippe,  107 
Trigone    (or    trigonum)    acustici.        See    Area 
acusl  ica. 

collateral,  248 

habenulae,  220 

hypoglossi,  127 

interpedunculare.    See  Fossa  interpeduncula- 
ris. 

olfactory,  266 

vagi.     See  Ala  cinerea. 
Trophic  unity  of  neuron,  51 
Truncus  corporis  callosi,  244 
Trunk,  sympathetic,  335,  346,  347,  348 
Tuber  vermis,  198,  201 

Tubercle  (or  tuberculum)  acusticum.     See  Nu- 
cleus,  dorsal  cochlear. 

anterior,  of  thalamus,  213 

cinereum,  122,  280 

cuneate,  121,  137 

olfactorium,  268,  282 

cf  Rolando.     See  Tuberculum  cinereum. 
Tufted  cells,  276 

Tiirck's  bundle.        See   Tract,  ventral  cortico- 
spinal. 

Uncus,  240,  269,  277 
Utricle,  193 
Uvula  vermis,  198 


Vallecula  of  cerebt  Hum,  197 

\  alve  of  Vieusscns.     Set    l  elum,  anterior  med- 

nll.ii  \ . 
Velum,  anterior  medullary,  125,  128,  155 
am  I.  urn.     See  Velum,  antei  ior  medullai 
interpositum.     See   Tela  chorioidea  ol   third 

venl  ricle. 
medullare,  anterius,  125 
inferius.      See     Velum    medullare,    p 

rius. 
posterius,  2<)2 

superius.    See  Velum,  anterior  medullary, 
transversum,  29,  -il 
Vena  terminalis,  214 

Ventrit  le  (oi  ventricles)  of  the  brain,  25,  26,  27, 
117 
development,  26,  33,  34 
in  the  dogfish,  27,  28,  30,  .-51 
fourth,  26,  118,  125,  126,  127,  128 
lateral,  26,   118,  246 
third,  26,  IIS,  223 
Ventriculus  lateralis,  26,  246 
terminalis,  81 

tertius.     See  Ventricle,  third. 

Vermis,  inferior,  197,  198 
superior,   167 

Vesicles,  cerebral,  primary,  24,  25 
optic,  225 

Vestibular  apparatus,  188,  189,  190 

Vicq   d'Azyr,  bundle   of.     See    Tract,  mamillo- 
thalamic. 

Yieussens,  valve  of.     See  Velum,  anterior  med- 
ullary. 

Visceral  innervation,  335 

Visual  apparatus,  225 
receptive  center,  292 

Yisuo-psyehic  area,  293,  294 

Vomiting,  mechanism  of,  331 

Wallerian  degeneration,  105,  106,  107 
Weight  of  brain,  301 

Worms,  nervous  system  of,  19,  20,  21,  22 
Wrisberg,  nerve  of.     See  Nervus  intermedius. 

Zone,  cortical.     See  Center,  cortical, 
ependymal,  37 
mantle,  37,  42,  196 
marginal,  37,  42,  196 


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